Chem 205L Final exam
As you ___ the wavelength of a psi electron making a transition state jump, the ΔE will ___.
increase, decrease ( they are inversely related)
the larger the isotope's mass, the __ the reduced mass and moment of inertia, which results in a ____ rotational constant and a ___ frequency
larger; smaller; lower
If substance A has a greater transition energy from n=0 to n=1 than substance B, electrons of which substance would absorb a light with a greater wavelength when they transition from n=0 to n=1?
substance B
For a normal mode to be IR active, there must be a change in the ______ . For a mode to be Raman active, there must be a change in the _________.
molecular dipole; polarizeability
what are two factors to consider in any measurement of infrared spectroscopy?
resolution; sensitivity
True/False. Computed quantum chemical harmonic vibrational frequencies accurately determine the various vibrational modes associated with molecules in comparison to their actual experimental frequencies.
false
true or false: The P branch represents only a transition between rotational energy levels and not between vibrational energy levels.
false
true/ false: Raman spec can show rotational transitions
false
Which dye would be orange?
500 nm
What is the wavelength of mass absorption for 1,1'-Diethyl-2,2'-Cyanine Iodide?
523 nm
If the given wavelength for a peak in the helium emission spectrum is 587 nm, what is the corresponding transition frequency (cm-1) for this peak?
587 nm x 1𝑚 / 109𝑛𝑚 x 100𝑐𝑚 / 1𝑚= 5.87 x 10-5cm →take reciprocal = 17036 cm-1
Calculate the length of a conjugated hexane chain when x=1.39*10^-10.
8.34*10^-10m
What is the approximate scaling factor that was used in this lab to make the Hartree-Fock calculations more accurate to the experimentally observed values?
.9
The molecular structure of 1,1'-Diethyl-2,2'-Cyanine Iodide (a conjugated dye) is shown below. When applying the quantum mechanical model to the molecule, what is its j-value?
1
1,1'-Diethyl-2,2'-Cyanine Iodide absorbs a green light while 1,1'-Diethyl-2,2'-Dicarbocyanine Iodide absorbs a red light. Which substance has electrons in bigger L?
1,1'-Diethyl-2,2'-Dicarbocyanine Iodide
At what angle to each other are the light source and detector placed in Fluorimeters?
90 degrees
1. All of the following apply to the Beer-Lambert law EXCEPT: a. incident light b. concentration c. pathlength d. molecular mass e. all of the above apply to this law
d
How does the rotational constant of HCl compare to that of DCl? A) it is higher due to its higher effective mass B)it is lower due to its lower effective mass C) it is lower to its longer bond length D) it is higher due to its shorter bond length E) no change
?
Which of the following is the highest energy wavenumber? 25773.1959 22371.3647 19960.0798 17035.7751 14992.5037 14164.3059 13736.2637
?
Which element has an emission spectrum that most closely matches that of the Sun? In parentheses is the number of electrons found in that element. a. Hydrogen (1) b. Helium (2) c. Hg (80) d. U (92)
D
Based on the PIB model, we have . Explain the meaning of and why and .
Each orbital allows two electrons. Thus, if we have electrons, it will take orbitals. Then, the occupied orbital with highest energy is , while the unoccupied orbital with lowest energy is . Therefore, the equation calculates the energy difference between the LUMO and HOMO.
2. Stokes shifts result from: a. vibrational relaxation b. very small wavefunction overlap of excited and relaxed states c. internal conversion d. a and c e. all of the above
d
Which of these molecules (HBr, H2, HCl, O2) would be IR active
Her and HCl, since these molecules have a permanent dipole moment and can be detected as they vibrate
as plotting data with Mathematica, which function will give a smooth line for the absorption vs. wavelength graph?
ListLLinePlot
How would you find the wavelength of max absorbance for a molecule from its absorbance spectrum?
Locate the tallest peak that corresponds to the highest absorbance, and given that its x-axis plots wavelength, find the coordinate of that peak and report its x-value.
why is a He atom more complicated than a 1 e- atom?
because it has both singlet and triplet E states where the e- are opposed or aligned differently
Which of these normal modes in CO2 is Raman active?
bend
Explain the following notations: 1s∞d←1s2p
The notation represents the transition between 1s2p orbital to the infinity high energy levels, which implies the ionization energy which completely removes an electron.
For the box length estimate (2j+5)x, what does j represent?
The number of double bonds in the polyene chain between two rings.
Describe the particle-in-a-box model. Under what conditions does it provide the most accurate estimation?
The particle-in-a-box model provides a better estimation for molecules with longer box lengths.
In which region(s) of the electromagnetic spectrum are excited photons found?
The photons are in the visible to ultraviolet regions.
What happens when complementary colors are combined?
White light is produced.
The absorption peak of a certain dye is 400 nm. What color does the dye appear to be?
Yellow
How would you use the answer in question three to help you find the Box Length (L) of a Cyanine dye?
You would use the equation E=(h2/(8meL2))(N+1) where N is the number of double bonds between the two ring structures multiplied by two then added to 4.
What factors affect ionization energy?
Zeff, electron shielding, atomic radius
Calculate the wavelength of a photon absorbed in the transition of n=3 to n=4energy level of a molecule.
(7h^2)/8mL^2
What are three advantages of Raman spectroscopy versus IR absorption spectroscopy?
1. Vibrations that might be weak in IR can be stronger in Raman. 2. Raman uses a small diameter laser beam, so one can use a very small sample to measure the spectrum. 3. Samples that are in aqueous solutions can be measured via Raman spectroscopy since there is no inference from water absorption.
Calculate the wavelength that corresponds to a energy level transition from 4 to 2 of the hydrogen atom in nanometers (Rydberg constant for Hydrogen = 4.39x10^7 m^-1)
121 nm
Which of the following transitions in a hydrogen atom would have the same energy? 1s2←1s2s 1s2s←1s3p 1s2←1s2p 1s3d←1s3p
1s2←1s2s and 1s2←1s2p have the same energy.
What do you need to include when counting pi electrons?
2 pi e- for each 2x bond and any lone e- pairs ( on N to say)
what are the spacings between the rovibrational lines of P and R branches
2B
What is the number of pi electrons in the system N?
2j+4
The molecular structure of 1,1'-Diethyl-2,2'-Dicarbocyanine Iodide (a conjugated dye) is shown below. When applying the quantum mechanical model to the molecule, what is its j-value?
3
A molecule strongly absorbs light at 625 nm. What is the energy gap between orbitals associated with this transition?
3.18 x 10-19 J
1,1'-Diethyl-2,2'-Cyanine Iodide's wavelength of the absorption peak is 525 nm. The change in energy of this dye using the equation, ΔE = hc/λ, is:
3.79 x 10-19 J
Which of the following is true about the "particle-on-a-line" model calculations seen in lab 1? A) the model gets increasingly accurate with a greater presence of nitrogen B) the model gets increasingly accurate with a longer conjugated carbon chain C) the model gets increasingly accurate with a lesser presence of nitrogen D) the model gets increasingly accurate with a shorter conjugated carbon chain
?
Which of the following molecules' stretching modes is Raman but not IR active? A) HF B) CO2 C) N2O D) H2SO4
?
Which of the following wavenumbers was found from the highest wavelength? 25773.1959 22371.3647 19960.0798 17035.7751 14992.5037 14164.3059 13736.2637
?
The following description is the definition of which phenomenon: electronic transitions happen on a time scale which is much shorter than nuclear motions, such as vibrations; therefore, vibronic transitions are more likely to occur if the two vibrational wave functions have significant overlap. Determines intensity pattern of absorption and fluorescence spectra a) Stokes shift b) vibration c) frank codon principle
A
A FTIR spectrum of a sample containing 2 isotopes shows that the P and R branch contains two peaks. Which isotope, the lighter or heavier one, will contain the highest frequency
A heavier molecule will correspond to lower vibrational frequencies.
Why does helium have a more complicated emission spectrum with more lines than hydrogen?
A helium atom has more possible transitions than hydrogen, where unlike hydrogen, different values of l(angular momentum)in the same principal quantum energy levels are no longer degenerate, thus giving more varied transitions between n1and n2.
Is the symmetric or antisymmetric stretch of carbon dioxide IR active?
Antisymmetric.
Below is the equation used to calculate ∆E of conjugated systems when its electrons absorb light. What do each of the variables (h, m, L, j) represent?
A: h = Planck's constant m = mass of an electron L = box length j = number of C-C double bonds in the chain
What are the different UV-Vis Spectroscopy applications (name one or two)
Absorbance can be used to obtain concentration of a certain sample or the identity of a compound by comparison of the absorption spectrum to literature spectrum.
If a substance absorbs light at approximately 510 nm (green), give the general wavelength of the color it reflects.
Absorbing green means it will reflect its complement; red. Red's wavelength is generally from 630-750 nm.
Can a vibrational mode of CO2 simultaneously be IR active and Raman active?
According to the rule of mutual exclusion, for molecules with a center of symmetry (capable of inversion symmetry), vibrational activity is mutually exclusive in IR vs Raman. So for these types of molecules, no vibrations can appear in both IR and Raman spectra.
Which of the does spectral intensity depend on for advanced IR spectrometry? a.) Sample concentration b.) Light intensity c.) Sample path length d.) Spectral line length e.) All of the above
All of the above. Note that the only way to increase the sensitivity for weak signals is to increase the intensity of the light or the response of the detector.
Calculate the number of vibrational modes in benzene.
Answer: 30 because benzene is nonlinear, so we use the formula 3N-6. Since benzene is C6H6, there are 12 atoms, so 3*12-6 = 30 modes.
Given the following transitions, which of the following is Stokes Raman scattering, and which of the following is anti-Stokes? A) Initial: V=1, Final: V=0 B) Initial: V=0, Final: V=1
Answer: A is anti-stokes, B is stokes
What is the transition in J state for the second peak on the R branch? a. J=1 --> J=0 b. J=1 --> J=2 c. J=2 --> J=1 d. J=1 --> J=2 e. J=1 --> J=3
Answer: D The R branch transitions increase the J state by increments of 1, so the second R branch peak would represent the transition from J = 1 to J = 2.
Why is the formula for calculating the number of modes have a 3 in front of N, such as in 3N-5 for linear and 3N-6 for nonlinear molecules? a. 3 is only for N=3, or when there are 3 atoms involved in the system (ex: CO2). b. 3 represents the degrees of freedom for motion of molecules in the x, y, and z directions. c. 3 is incorrect. The formulas only multiply by 2.
Answer: b because the 3 represents the degrees of freedom of motion of a particle in 3-dimensional space and thus 3*N vibrational modes prior to subtracting out translation and rotations.
Circle all Raman active modes in a tetrahedral molecule like methane: a. Antisymmetric stretch b. Symmetric Stretch c. Antisymmetric bend
Answer: b. Symmetric Stretch since the symmetric mode has a change is polarizability and bond length, it is a Raman active mode.
Complete the following statement for the first and second blank: Stokes scatter occurs when the photon emitted is __________frequency than the incident photon, while Anti-Stokes scatter occurs when the photon emitted is ___________than the incident photon. a. Higher and higher b. Lower and lower c. Lower and higher d. Higher and lower
Answer: c. Lower and Higher
Why is the Stokes component more intense than the Anti-Stokes component?
Anti-Stokes scattering results from excitation from v=1, where the population is lesser than that of v=0, where Stokes scattering takes place. Thus, the resulting line is naturally weaker than the Stokes line due to the excitation of a smaller population.
Why is it important to conduct Raman spectroscopy in a dark enclosure?
Any stray light from the room could add to the background noise in the spectrum.
What are three key parts of a good abstract?
Any three of the following: what you did, purpose of experiment, conciseness, clarity, important numerical results, summary of non-numerical observations, and no typos or grammatical errors.
What is a possible wavelength of light absorbed by a green object?
Any wavelength that's not 525-572nm
If a substance is blue, estimate the length of the wavelength at which maximum absorption occurs when analyzed via UV-Vis spectroscopy.
Around 610-630 (Orange is opposite of blue on the color wheel)
Which is an approximation that the Particle in a Box model makes about the conjugated system?
Assumes electrons don't interact with each other
Which of the following approximations does the particle in a box model not make?
Assuming borders with finite potential energy
Which of the following is an acceptable way to improve IR spectrometry? a.) Decreasing frequency range b.) Increasing frequency range c.) Increasing spectral acquisition rate d.) A and C e.) B and C
B and C. The only way to advance IR spectroscopy is to increase frequency range increase spectral acquisition rate, increase resolution, or increase sensitivity.
You are handed an absorption spectrum with a peak at a wavelength of 560nm. If the solutions in two cuvettes labeled A and B appear yellow and violet, respectively, which cuvette does the spectrum probably belong to?
B, since yellow light is absorbed best in the 500-600 nm range, meaning a solution with an absorption peak at 560nm should absorb yellow light very well and appear violet (its complementary color).
Which of the following Molecular vibrational frequencies corresponds to the mid-IR spectrum? 400-10 cm−1 4000-400 cm-1 6000-4000 cm-1 10000-6000 cm-1`
B, the mid-IR spectrum corresponds to a vibrational frequency range of 4000-400 cm-1 (2.5-25 μm)
Which of the following does NOT describe the components and structure of an abstract?
Background Info
An electron in a system that transitions from an energy level down to the second energy level can be associated with the _______ series (CIRCLE: Balmer series OR Paschen series) and should emit ________ (CIRCLE: UV light OR visible light)
Balmer Series and visible light
Why can an electron only move up by one energy level at a time?
Because a photon has a unit of 1 angular momentum therefore it can only go up by 1 energy level
What are cyanine dyes commonly used for?
Cynanine dyes are commonly used to label biological molecules of interest for biophysical spectroscopy and microscopic imaging.
What color light would a molecule of H emit if it releases energy of frequency 610 THz?
Blue
If a wavelength of 450 nm is absorbed, what color is reflected and what color is absorbed?
Blue is absorbed, Orange is reflected.
Stokes, the scientists who developed the Stokes shift, also developed which term? a) Emission b) Absorption c) Phosphorescence d) Fluorescence
D
Why is there a bigger difference in frequency betweenCH -CD stretches for benzene and deuterated benzene in comparison to their respective ring stretches?
CH and CD stretches directly involve movement with the isotopic hydrogens, and thus the differences resulting from differing reduced masses affect the vibrational frequencies most strongly -resulting in a much higher frequency CH stretch and lowerfrequencyCD stretch. For ring stretches, the carbon ring skeleton is conserved across both molecules, and while there is some effect from expanding against different isotopes of hydrogen, it is more indirect and thus, ring stretch frequency differences between benzene and deuterated benzene are not as great.
Why do the calculated frequencies overestimate observed frequencies?
Calculated frequencies use the harmonic approximation
Match the following real-life system to the model used to explore the phenomena in lab Real-life systems: i. Vibrations of diatomic ii. Conjugated dye iii. Rotations of diatomic Models: iv. Harmonic oscillator v. Particle on a line vi. Rigid rotor
Conjugated dye= Particle on a line (ii. and v.) Vibrations of diatomics= Harmonic oscillator (i. and iv.) Rotations of diatomics= Rigid rotor (iii. and vi.)
Would you expect the ω0 of H79Br or H81Br to have a larger value?
H79Br
What is the selection rule for the P branch, DJ=?
DJ= -1
At the Q branch, delta J=?
DJ=0
How can a student fix an absorption spectrum in which the maximum goes off screen?
Dilute the solution and measure again.
Use the absorption spectrum and Planck_ Einstein equation to determine the E of the photon released by the substance at the greatest absorption.
E= hv=hc/lambda=3.8E-19 J
Given the maximum wavelength of a cyanine dye, how would you calculate the total Energy?
E=(h*c)/lambda
Which of the following is NOT one of the assumptions made by the 1-D Particle in a box model for real molecules?
Electrons are fixed in the bond between two atoms and do not move around.
Choose either equals or not equals for Hydrogen and explain: E(2s) equals/not equals E(2p)
Equals because the orbitals are degenerate.
Why is methanol a good reference to use for the UV-Vis spectra?
It is clear so no peaks would show up on the spectrum.
Assign the correct change in quantized energy value for excitation, Rayleigh and Raman transitions.
Excitation: v=0 --> v=2, Rayleigh: v=2 --> v=0, Raman: v=2 --> v=1.
True/ False. For a vibration to be Raman active it requires for the vibrational mode to have a change in dipole moment and polarizability.
False; Raman spectroscopy depends on the vibrational mode changing the molecule's polarizability.
True/False → In multi-electron atoms, energies depend only on the principal quantum number.
False
The Benzene irrep E1u is ungerade, which is both IR active and Raman active. True or False? True False
False because Benzene's point group is D6h and within this point group the only irreps that are Raman active are gerade irreps which are left unchanged by the operation of inversion. This means they are symmetric. So A1g is Raman active but E1u is not Raman active but is still IR active. A vibration is IR active, Raman active, or both, and totally symmetric vibrations are always Raman active.
A Q branch transition is allowed for diatomic molecules. True False
False because a Q branch transition is one where ∆J = 0, and is forbidden for diatomic molecules.
True or false, in Raman spectroscopy, photons are absorbed or emitted.
False, Raman spectroscopy is a scattering technique, so NO photons are being absorbed or emitted.
True/ False: For an infrared spectrum, the spectrum is a result of the excitation of only the vibrational levels associated with the atoms.
False, the infrared spectrum is a result of the excitation of both vibrational and rotational levels. Both of these factors, rotations and vibrations contribute to the total change in energy for each transition in a spectrum.
True or false: An absorption spectrum shows you discrete bands of the color light absorbed by the substance.
False. An absorption spectrum would show you all the color light not absorbed and there would be color absent (seen as black) in "bands" where the absorbed light was
Describe some of the advantages of Fluorescence Spectroscopy.
Fluorescence spectroscopy is great for complex samples, because only few molecules can fluoresce. In other words, the technique can be highly specific and accurate (avoids contamination by other things). In addition, there is barely any interference from other light sources when a sample is placed at a 90 degree angle to the incoming excitation beam.
In the Rydberg equation, how would you represent ionizing an electron?
For the general hydrogen Rydberg equation, ṽ= RH(1/ 𝑛1(^2)−𝑛2(^2)),you would set n2= ∞and n1= 1 to represent the removal of one electron from its ground state (in gas phase). The same generally applies for helium, extrapolating n2= ∞ and n1=2 and adding the given transition frequency of 1s2→1s 2p energy to the calculated energy of 1s 2p →1s ∞d.
When analyzing a rovibrational spectrum using H35Cl and H37Cl, which isotope will have the lower frequency peaks?
H35Cl will have lower frequency peaks for the transitions due to its smaller/ reduced mass than H37Cl.
When analyzing a rovibrational spectrum using samples of H35Cl and H37Cl, which isotope will appear with the highest absorbance value?
H35Cl will have the highest absorbance since there will be more H35Cl in the sample than H37Cl in the sample.
The absorption peak for a molecule is at 625 nm. Which orbitals is the electron transitioning from, and which orbital is it transitioning to?
HOMO to the LUMO
What are two hydrogen- like atoms that can be used for this equation in order to find the atom's wavenumber?
He+ and Li+
As we increase the length of a PIB model (i.e. increasing the size of the conjugated dye), so we expect to see a peak absorption at a higher or lower wavelength?
Higher. The length of the PIB model and its peak absorption correlates one another.
What is the main difference between Raman and IR spectroscopy in terms of how they detect a molecule's vibrational movements?
IR spectroscopy depends on absorption and emission of photons,which will show IR peaks for molecules if their vibrational modes induce a change in dipole moment,while Raman utilizes a scattering technique that will show peaks if there is a change in polarizability. Thus, Raman spectroscopy is non-destructive as there is no actual absorption by the sample.
IR spectroscopy is a(n) (absorption/ scattering) technique while Raman spectroscopy is a(n) (absorption/ scattering) technique.
IR spectroscopy is an absorption technique while Raman spectroscopy is a scattering technique.
If a molecule has strong covalent character, which would be weaker, IR or Raman?
IR would be weaker
What are the effects of "quenching" on the fluorescence of a molecule?
Impurities within a fluorescent sample "quench" the absorbed energy of the fluorescent molecules kinetically and prevents fluorescence from the molecule when that energy is lost to the impurities.
Based on the PIB model, we develop a model to approximate the box lengths: Please describe what L represents.
In this equation, represents the number of double bonds along the polyene chains, between the aromatic rings.
When a molecule undergoes non-resonant fluorescence, which of the following is true? It will emit a higher energy photon than it absorbed It will emit a lower energy photon than it absorbed It will emit a blue-shifted photon It will emit a red-shifted photon
It will emit a lower energy photon than it absorbed It will emit a red-shifted photon
what does m equal for R branch transitions
J" + 1
Using the Particle in a Box model and the equation L = (2j + 5)x , estimate the "box length" of a molecule comprised of a conjugated double bond chain with six C-C double bonds. (Assume the length of C-C bond of order 1.5 is 1.39 x 10-10 m).
L = (2(6) + 5)(1.39 x 10-10 m) = 2.363 x 10^-9 meters
Knowing that N= 6 and Ephoton= 3.80E-19 J, solve for the box length of the molecule
L= ((h^2*(2j+5)/ (E*8* 9.109E-31kg))^1/2= 1.05E-9 m
Which of the following is the most polarizable? a. Large atoms with covalent bonds b. Large atoms with ionic bonds c. Small atoms with covalent bonds d.Small atoms with ionic bonds
Large atoms with covalent bonds
Do heavier or lighter molecules have IR peaks at higher frequencies?
Lighter.
Circle the right answers: Stokes lines are (higher/the same/lower) in frequency than the incident light and thus appear at (longer/the same/shorter) wavelengths.
Lower; longer.
What are the effects of aromatic rings on box length?
Makes them longer
What property does a molecule with a center of symmetry exhibit?
Mutual exclusion
Which of the following is an effect of quenching? a.) Increase fluorescence lifetime b.) Non-radiatively deactivating the excited molecule through collisions c.) Increase the intensity of a sample d.) Complex formation that aligns excited molecule and solvent dipoles e.) It has no effect on quantum yield from a fluorophore
Non-radiatively deactivating the excited molecule through collisions is the only option that correctly identifies the impact of quenching.
Molecules can undergo what kinds of transitions?
Nuclear, transitional, vibrational, and electronic
Which of the following diatomics is Raman active but not IR active? a.) O2 b.) NO c.) HCl d.) NaCl e.) None of the above
O2 is Raman active but not IR active because O2 is a homonuclear diatomic with no permanent dipole moment therefore it is not IR active.
When would one increase sensitivity in an FTIR experiment and how would one do so?
One would increase sensitivity in FTIR to detect weaker spectral features and the way to increase sensitivity is by increasing the light intensity or the response of the detector.
On a transition spectrum with P and R branches, which side are these branches on, and which branch starts with 0
P branch on left, R branch on right and starts w 0
what is the primary difference between the existence of "P Branches" versus "R branches"?
P branches represent a lowering of energy while the R branches represent raising energy transitions.
Why are electrons in the Balmer, Paschen, and Lyman series promoted to higher levels and the higher orbital shell (2s → 2p) instead of promoting electrons within the orbital shell (2s → 3s)?
Photons which contain the energy to promote the electrons to a higher level have a value of angular momentum. This causes the electrons to be not only promoted to a higher energy level but also a higher orbital shell.
Which branch of the rovibrational spectrum corresponds to the vibrational quantum number v increasing by 1 and the rotational quantum number J increasing by 1?
R branch
How does Raman spectroscopy measure frequencies vs IR spectroscopy?
Raman spectroscopy observes the vibrations of the molecule while IR measures what wavelength the molecule absorbs.
If a bond has more covalent character than ionic, is it stronger in IR or Raman spectroscopy?
Raman.
What is the difference between Resolution and Sensitivity?
Resolution is how well two peaks can be distinguished from each other while sensitivity deals with the intensity of a measurement and is the degree of reaction to an incoming signal
Which of the atomic/molecular transition(s) correspond(s) to the absorption of light that cannot be detected by UV-Vis spectrometer?
Rotational transitions and Vibrational transitions.
Which of the following relates to symmetric vibrations? (Objective 1) a. Weak Raman and strong IR b. Weak Raman and weak IR c. Strong Raman and strong IR d. Strong Raman and weak IR
Strong Raman and weak IR
Name one benefit and one limitation of using Raman spectroscopy over IR spectroscopy
Since Raman spectroscopy detects vibrational modes through change in polarizability, highly symmetrical molecules with symmetrical vibrational activity that are not IR active can instead be detected and studied through Raman. Generally, it can be difficult to obtain high resolution Raman spectra since the use of UV-VIS light makes it hard to achieve high resolving power.
Why is the potential energy over a conjugated system relatively constant?
Since the electrons are resonating over the entire conjugated system, the energy is evenly split over the conjugated system.
Why is Raman particularly well suited for the study of biological samples?
Since water is not a good Raman scatterer, Raman spectra of samples in aqueous solution can be obtained without interference from water absorption. This makes Raman particularly well suited for the study of biological samples.
What are the general selection rules for symmetric, asymmetric, and bending vibrations?
Symmetric Vibrations will have Strong Raman and weak IR. Asymmetric vibrations will have weak Raman and weak IR. Bending vibrations will have weak Raman and Strong IR as well.
Which type of motion (stretching) does IR spectroscopy focus on?
Symmetric and antisymmetric stretching
Which peak would not be seen on a CO2 IR spectrum? Which would not be seen on a CO2 Raman spectrum?
Symmetric stretch; asymmetric stretch
How does the energy of a photon emitted in the UV region compare to the energy of a photon emitted in the visible light region?
The UV photon will have more energy.
Why would high concentrations of fluorescent molecules affect the calibration curve?
The analyte begins to cause absorption and fluorescence overlap.
Using the equation (With j being the number of double bonds in the polyene chain and x is the C-C length bond order) ,why is it expected that the bond length will be slightly longer than simply counting the bonds in the conjugated chain?
The dyes have aromatic rings at the ends of the box which increases the effective length of the box.
Why is the Balmer series expected to best follow the one-electron theory?
The excited electron doesn't significantly penetrate into the 1s electron cloud and it interacts with the nucleus as if it has a +1 charge due to the 1s electron shielding.
What features of harmonic models prevent it from being a completely accurate model?
The harmonic model assumes that all energy levels are equal in distance
In this lab, what is one major reason why there were significant disparities seen among the experimentally recorded Stokes Shift values and the calculated Stokes Shift values?
The major reason why we see the significant difference is due to anharmonicity.
Why does a highly conjugated system absorb photons of lower energy (higher wavelength)?
The more conjugated a system is, the lower the energy gap between the atomic orbitals. This is because a conjugated system offers stability as the electrons are delocalized through adjacent pi orbitals. This stability is evident through the fact that highly conjugated systems absorb photons of lower energy, as the energy gap between HOMO and LUMO is decreased.
What causes the splitting of the orbitals in a helium atom?
The shielding effect, which is the electron-electron repulsion between the two electrons.
What is the significance of the slope of the plot of fluorescence intensity vs. increasing concentration of an analyte with fluorescent characteristics?
The significance is the linear relationship between fluorescent intensity and concentration of a solution. Through such a linear relationship, concentration of an unknown sample can be determined!
You've just measured the absorption spectrum of a cuvette full of 1,1'-Diethyl-2,2'-Carbocyanine Chloride solution and your maximum absorbance is 3.7. What went wrong and how can you fix it?
The solution is not diluted enough, so the absorption signal was saturated. Another dilution should be performed so that the absorbance peak is less than 1 for the most accurate spectrum.
What is the range of wavelengths that is associated with the visible light spectrum?
The visible light spectrum is a portion of the electromagnetic spectrum, with range: 380 - 700 nm.
What's the difference between stokes and Anti-Stokes shifts in a Raman Spectrum?
These are the two types of scattered light. Stokes is the (ν0 - νm) frequency while anti-stokes is the (ν0 +νm) frequency where vm is the vibrational frequency of the molecule related to the energy difference between the two vibrational states of interest.
Why do the transitions from Lab 2 end at n1 = 2?
These transitions are part of the Balmer series
Why are some wavelengths missing in a spectrum (Fraunhofer lines)?
They are absorbed by atoms in the atmosphere
Why are calculated Raman frequencies usually larger than the actual frequencies?
They are found using the harmonic theory while reality is anharmonic.
A student is comparing the spectral lines of hydrogen and helium and notices the helium spectra lines are invisible to the human eye. Why are the helium spectral lines invisible?
This is because the Helium transition occur in the UV region of the electromagnetic spectrum and are unable to be seen with a naked eye (unlike the transition for Hydrogen spectral lines).
What are some assumptions made when using the PIB model that would result in discrepancies when using it to estimate real systems?
To list a few options: we assume there are potential energy walls that rise to infinity and that there is no electron interaction that could raise potential energy above zero, we model three dimensional movement in a one dimensional system, and we apply generalized estimations of box length calculated from molecular properties - specifically, such as L=(2j+5)x for cyanine dyes.
How can we explain the widening of emission lines in the Helium emission spectrum?
Transitions are non-singular since 2 electron species have non-degenerate energy levels due to e-e interactions.
In the Balmer series, why does the angular momentum shift by one between transitions, ie. s →p, p →d?
Transitions change by one value of l (angular momentum) since the photons with the specific energy to excite electrons between the upper and lower states in the Balmer Series have the value of 1 l.
Is a triplet or singlet state more stable, and why?
Triplet because it has less electron-electron repulsion and lower energy
True or false? The Stokes shift is dependent on both the vibrational relaxation and the solvent reorganization.
True. The Stokes shift is a result of an excited fluorophore dipole surrounded by solvent molecules. The dipole moments in the fluorophore and the solvent are temporarily misaligned and it results fluorescence before the dipoles realign after vibrational relaxation.
True or false? Transitions are more likely if there is more overlap with the vibrational wave functions of the two states.
True. This has to do with the fact that molecules make vertical transitions and absorbing and emitting photons is a fast process, so the nuclei don't have time to move. The Franck Condon principle explains this via a parabolic graph that shows the energy and nuclear coordinates.
What type of instrument is used for calculating absorption spectroscopy in this experiment? (lab 1)
UV-VIS Spectrophotometer
What is a type of light that causes molecules to split? How and why does this occur?
Ultraviolet light can cause this because when its photons excite a molecule, they can cause electrons to be knocked out of bonds. This happens because UV light is high energy and causes intense shaking of the electrons that is powerful enough to split bonds.
What does the αe value signify?
Vibration-rotation constant
What molecular movement corresponds with the absorption of IR light? Microwaves?
Vibration. Rotation.
Stokes shift comes from:
Vibrational Relaxation Internal conversion
What kind of transitions does a Fourier transform infrared spectrometer detect?
Vibrational and rotational transitions where different frequencies are specific to specific functional groups.
Match the following electromagnetic radiation with the effect it has on the motion of molecules: Electromagnetic Radiation: a. Visible b. Infrared c. Microwave Molecular Motion: d. Symmetric and antisymmetric stretches, bending e. Transitions of electrons f. Radial rotations
Visible = Transitions of electrons (a and e) Infrared = Symmetric and antisymmetric stretches, bending (b and d) Microwave = Radial rotations (c and f)
Would an emission of energy with a frequency of 550 THz fall in the visible light or UV spectrum?
Visible Light
Why is Raman spectroscopy useful in biological aspects?
Water is not a good Raman scatterer, so it is useful in observing molecules in biology where water is prevalent.
If green is reflected, what was the absorbed wavelength and color?
Wavelength 650 and red is absorbed.
For the transition frequency ratio of ṽH,4→2 / ṽH,3→2, which ratio corresponds best for these experimental helium transition values? 20319 𝑐𝑚−1/ 14147𝑐𝑚−1 or 19938𝑐𝑚−1/ 14973𝑐𝑚−1 or 20319𝑐𝑚−1/14973𝑐𝑚−1?
We know that the standard ratio ofṽH,4→2 / ṽH,3→2 = 20576𝑐𝑚^−1/ 15244𝑐𝑚^−1= 1.35. Thus, the last ratio, 20319/14973, comes closest to 1.35.
Highly symmetrical molecules with IR active modes tend to have _____ intensity.
Weak or low
What are the two basic points that should be addressed in an Abstract?
What was done in terms of a quick experimental summary and what was found, stating the most important conclusions with key numerical results.
When will an electron be excited from a lower energy state to a higher energy state?
When a photon is absorbed corresponding to a discrete energy value
In what specific instance does internal conversion occur?
When the molecule relaxes after being excited, it will undergo internal conversion if there is another electronic state that is lower in energy AND has the same spin multiplicity as the excited state.
When does a Q branch for a diatomic molecule show?
When there is electronic angular momentum in the ground electronic state.
The 1-D particle in a box model helps visualize and evaluate the relationship between electrons and energy. However, simplifies certain aspects of reality that make it an imperfect model of an actual electron's activity in a conjugated system. In what way(s) is the 1-D particle in a box model flawed?
a) An electron exists in three dimensions, and the model only takes into account one dimension movement. b) The model does not take into account the fact that the electron in question would normally be in contact with other electrons, affecting its potential energy. c) Makes an approximation of "box length" that yields infinite potential energy for the wavefunction should it exceed the box length boundaries.
An electron in transition from n = 2 to n = 4 will _____ a photon with a wavelength that is ____ times greater than an electron transitioning from n = 3 to n = 5 from the same molecule.
absorb; 1.33 (because you can compare the equations λ = 1 / (Δn2) since they are from the same molecules and everything else stays constant.)
What factors does reduced mass affect
affects I and B, thus affects the change in E for transitions
the absorption/ emission of visible light corresponds to which series
balmer series
The wavelength of visible light a substance absorbs is 600 nm. The color of this substance is:
blue
For the hydrogen atom: a) Energies do not depend on n, the principal quantum number. b) Energies depends on n (principle quantum number) and l (angular momentum quantum number). c) Energies depend only on n, the principal quantum number. d)Energies do not depend on n (principal quantum number) or l (angular momentum quantum number).
c- Energies depend only on n, the principal quantum number.
Which is true about Raman Spectroscopy? a. It is based on change in dipole b. It requires a net dipole in the molecule c. It is based on a change in net polarizability d. It is based on a change of molecular structure
c. It is based on a change in net polarizability
How do quenchers affect the fluorescence of a molecule? a) They prevent them from fluorescing b) They deactivate the electronically excited molecule c) They prevent them from absorbing radiation d) All of the above
d
After measuring a sample dye in the spectrometer, you see that the resulting graph is illegible, with what appears to be a peak that is so large it doesn't fit on the screen. What seems to be the problem?
d) The sample has too high of a concentration, resulting in a saturated and illegible reading.
Since frequency is dependent on reduced mass, frequency will (decrease/ increase) for deuterium.
decrease
If the conjugated system is extended, the HOMO-LUMO energy gap is expected to (decrease/ increase), which would then (absorb/ emit) the light at a (shorter, longer) wavelength.
decrease; absorb; longer
If a general molecule with given L=1.39 x 10^-9 m absorbs a photon that excites an electron from its 5th energy level to its 7th energy level, what is the ∆E? The mass of an electron is 9.11 x 10^-31 kg
delta E= ((h^2)/(8meL^2))*(nf^2- ni^2)= 7.48E-19 J
What is a limitation of IR spec but not Raman spec
depends on change in dipole moment for the molecule
What kind of species are not detected by infrared spectrometry?
diatomic molecules with no dipole (Cl2)
True or false: It is okay for an abstract to have three or fewer grammatical mistakes because the science community won't focus on them.
false
True or false: Virtual states reached by Raman spectroscopy are solutions to the Schrödinger Equation.
false
True or false: You should always use all of your data points when calculating a line of best fit for a calibration curve.
false
what type of transition is occurring depicted at the max wavelength of a UV- Vis absorption spectrum measured for a cyanine dye?
electronic
Which type of spectrum shows excited gas going to its ground state?
emission
Stokes Shift is the difference between _________ wavelength and ___________ wavelength.
emission and absorbance.
Hydrogen's Balmer series of _____________(emission/absorption) _______(ends/begins) at level __(2/3/4/5/6).
emission, ends, 2
True/ false: Raman spectroscopy is more accurate than IR spectroscopy
false
T/F: Absorption and emission spectra give the same exact information.
false
T/F: Benzene has vibrational modes that are both IR- and Raman-active.
false
T/F: On an emission spectrum, a peak found at 600 nm would correspond to a higher energy level transition than at 300 nm.
false
T/F: The Stokes' Shift is the difference between the molecule's fluorescence and phosphorescence emission wavelengths
false
True or False, spectral line strengths can be changed experimentally.
false
True or False: The laser used in Raman spectrometer must input light over a range of frequencies.
false
True or False: when a molecule undergoes internal conversion, it emits radiation.
false
T/F: The Rydberg equation would be an accurate description of the ionization energy for S
false, The Rydberg equation is an accurate descriptor of ionization energy for electron systems that do not penetrate the 1s electron cloud and systems where the 1s electron and nucleus can be considered as one system. The effective nuclear charge as well as the electron repulsion and electron shielding lead to penetration of the 1s electron cloud by the excited electron.
T/F: If a photon contains slightly more energy than the ΔE between energy levels 1 and 3, the electron can absorb that photon and jump to n=3.
fasle
If 720nm is absorbed, what color is reflected/ seen?
green
If the maximum absorption of a substance occurs at a wavelength of 650 nm, what is the color of the substance being analyzed? (Redlight has a wavelength around 650 nm.)
green
how do different isotopes rank in frequency, low to high freq?
highest frequency corresponds to the isotope with the highest number/ mass:
if a molecule has both IR and Raman spectrum at the same frequency, which of the following can you expect
if the raman is strong then the IR will be weak
Which part of the electromagnetic spectrum do rovibrational transitions occur in?
infrared light
For Benzene, if a mode is IR active, it will (not be, be, sometimes be) raman active
it will not be raman active
What are the main components of an abstract?
methods and results
1. Non-resonance fluorescence means the molecule will emit a photo with ______ (more/less) energy than the excitation photon.
more
In a hydrogen atom, which quantum numbers are required in order to determine energy level of its electrons?
n
Which of the following Balmer Series transition values for Hydrogen would have the highest frequency? a.) n → 2, n → 3 b.) n → 2, n → 4 c.) n→ 2, n→ 5 d.) n→ 2, n→ 6
n → 2, n→ 3
Which transition in Balmer series leads to an absorption of a photon that corresponds to red visible light? n1 = 2 → n2 = 3 n1 = 2 → n2 = 4 n1 = 2 → n2 = 5 n1 = 2 → n2 = 6
n1 = 2 → n2 = 3; The correct answer is A because a photon that corresponds to red visible light is a low energy photon since red is the lowest frequency visible light wavelength. The jump between n1 = 2 → n2 = 3 is the shortest jump, meaning that the electron needs to absorb the least amount of energy to be promoted to the 3rd energy level out of all of the transitions in the Balmer series. This low energy
What type of atomic and molecular transitions correspond to the absorption and emission of ultraviolet light?
n=4 -> n=1
Which of the following Balmer Series transition values would have the highest wavelength? a.) n → 2, n→ 3 b.) n→ 2, n→ 4 c.) n→ 2, n→ 5 d.) n→ 2, n→ 6
n→ 2, n→ 6
In what region of the electromagnetic spectrum is the changes in the vibrational energy of HCl detected?
occurs in the infrared region at lower energy
Raman spectroscopy measures the change in a molecule's ____ while IR spectroscopy measures the _____.
polarization; transition dipole moment
What color is the n=3 to n=2 transition of hydrogen?
red
Quenching ________ (reduces/increases) the quantum efficiency of a sample.
reduces
What does the Zeff term address in the Rydberg equation for helium?
shielding
Is a wavelength falling in the UV range smaller or larger than a wavelength falling in the visible light range?
smaller
Given the equation Zeff= Z- σ, How will Zeff compare to an atom with an atomic number of 5 vs an atomic number of 30?
the atom with the atomic number of 30 will have a Zeff value that is higher than the Zeff value for the atom with the atomic number of 5.
Using a plot and best fit line, how do you calculate Zeff
the slope of the plot =Zeff^2* Rhe, so divide slope by Rhe, then take the square root to calculate Zeff
Fluorescent molecules can be quenched by: 1. The solvent 2. External light sources 3. Increasing temperature
the solvent
are the stokes lines lower of higher in frequency than the incident light and do they appear at longer or shorter wavelengths?
the stoke lines are lower in frequency than the incident light and thus appear at longer wavelengths
why is the stokes component usually measured as opposed to the anti stokes component
the stokes component is more intense than anti stokes
where does the vibrational E of HCl occur
the vibrational E of HCl occurs at lower E in the infrared region of the spectrum
why does the emission spectrum of helium have more emission lines than hydrogen
there is orbital splitting with a 2 e- system; there are 2 types of excited states for helium (singlet and triplet); Because the orbitals at the same energy levels are not degenerate in helium, so more transitions are possible.
Which is predicted to have more energy, a singlet or triplet state, and why?
triplet state more stable since in a singlet, there is more electron- electron repulsion meaning the singlet will have a higher energy.
T/F Quenching reduces the quantum efficiency of a fluorescent sample.
true
True or false: The one-dimensional particle-in-a-box model gets better at predicting as the box length increases.
true
True/ false: raman can detect some vibrational modes in a molecule that IR cannot
true
true or false: The R branch represents a transition of J= +1
true
true or false: the ratios for the transition frequencies for H and He cation are the same
true
which of the molecular transitions correspond to the absorption of infrared light
vibrational transitions
An excited electron emits a violet light while another excited electron of the same atom emits a green light as they transition to ground state. Which electron has a bigger transition of energy levels?
violet light emitting electron
the balmer series corresponds to ____ light, Lyman series to ____ light, and Paschen series to ____ light
visible, ultraviolet, infrared
What color is the chemical compound if it does not absorb any of the wavelengths in the visible spectrum of light?
white
If a substance absorbs a wavelength of 400 nm most, what color is it?
yellow
The wavelength of visible light a substance absorbs is 400 nm. The color of this substance is:
yellow
What cannot be calculated given the wavelength corresponding to the max absorbance signal and a picture of the conjugated molecule?
∆E, Frequency, Box Length
