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Unformatted text preview: MIT OpenCourseWare 5.80 Small-Molecule Spectroscopy and Dynamics Fall 2008 For information about citing these materials or our Terms of Use, visit: MASSACHUSETTS INSTITUTE OF TECHNOLOGY Chemistry 5.76 Spring 1976 Problem Set #3 1. (a) Write out the electron configurations for the molecules O+ , O2 , O− , and O2− . 2 2 2 � � − (b) Determine the ground-state term symbols M Λg,u ± for O+ , O2 , O2 , and O2− . If there are two or more 2 2 low-lying states, select one as that of the ground state and justify your selection. 2. (a) Using symbols appropriate to the separated-atom approximation to a molecular orbital, write the elec­ tronic configuration of lowest energy for the diatomic species C2 , NO+ , and S2 . � � (b) Write the term symbols M Λg,u ± for all the electronic states derivable from the configuration of lowest energy for all three species. Which of these states will have non-zero magnetic moments? 3. Gaseous HCl is normally a 3 : 1 mixture of H35 Cl and H37 Cl. To a high approximation, the rotational energy levels of such diatomic rotators are E ( J )(in cm−1 ) = J ( J + 1) B − J 2 ( J + 1)2 D where B, the rotational constant, is larger by a factor 1.0015 for H35 Cl than for H37 Cl, and the centrifugal distortion constant D is the same for both molecules within the error of its measurement. (a) Derive an expression for the separation of the pure rotational absorption lines of H35 Cl and H37 Cl as a function of J� , the J–value for the upper state (ΔJ = J� − J�� = +1). (b) What is the spacing in cm−1 of the two lines for which J� = 10? 4. The “transition moment,” or the probability of transition, between two rotational levels in a linear molecule may be assumed to depend only on the permanent electric dipole moment of the molecule and thus to be the same for all allowed pure-rotational transitions. In the pure-rotational emission spectrum of H35 Cl gas, lines at 106.0 cm−1 and 233.2 cm−1 are observed to have equal intensities. What is the temperature of the gas? The rotational constant B for H35 Cl is known to be 10.6 cm−1 , and the ratio hc/k has the value 1.44 cm·deg. 5. What would happen to the Birge-Sponer extrapolation scheme for a molecular potential correlating with ionic states of the separated atoms? 5.76 Problem Set #3 Spring, 1976 page 2 6. The ground state and a low-lying excited electronic state of the BeO molecule have the following properties Term symbol Electronic energy, T e /cm−1 ωe /cm−1 ωe xe /cm−1 re /10−8 cm 1 Σ+ 1Π 0 1487.3 11.8 1.33 9405.6 1144.2 8.4 1.46 (Note that the electronic energy T e is the energy from the minimum of one curve to the minimum of the other; this is not equal to the vibrational origin of the 0 − 0 band.) (a) Construct a Deslandres table of the vibrational band origins of the 1 Π − 1 Σ+ system, for v�� = 0 through 3 and v� = 0 through 5. Which of these vibrational bands would you expect to be the most intense, when the system is observed in absorption? Comment on the relative intensities that you would expect for the other bands in your table. (b) In the rotational structure of the individual vibronic bands in this system, what branches would you expect to observe? In which branch would you expect to observe a band head? Calculate the transition in J that will give rise to a line at the band head, and the distance in cm−1 from the band head to the vibrational band origin. (c) What would you guess about the MO configurations corresponding to these two states? (HINT: Note that BeO is isoelectronic with C2 , so that the MO’s may be expected to be somewhat similar, except that the g-u property will be lost, and the orbitals will be distorted toward the higher nuclear charge of the O-atom.) Would you suspect the presence of any other excited electronic states below the 1 Π state? If so, what would its term symbol be? 7. The following bands are observed in the second positive system of nitrogen (units are reciprocal centimeters corrected to vacuum): 35,522 cm−1 35,453 33,852 33,751 33,583 32,207 32,076 31,878 31,643 30,590 30,438 30,212 29,940 cm−1 29,654 29,010 28,819 28,559 28,267 27,949 27,451 27,226 26,942 26,621 26,274 25,913 cm−1 25,669 25,354 25,003 24,627 24,414 24,137 23,800 23,414 23,016 �� � Arrange these in a Deslandres table, and find values for ω�� , ωe xe , ω� , and ωe xe . Important Suggestion: e e Look at the pattern of bands first, before doing anything else. Do any natural groupings seem to suggest 5.76 Problem Set #3 Spring, 1976 page 3 themselves? It may help to draw a “stick spectrum” of the band origins, to scale, in order to pick out these patterns. Remember that bands having the same Δv fall along diagonals on the Deslandres table.) Is there any suggestion of a cubic term in either state? If so, derive an expression for the third difference, � �3 Δ3Gv+1/2 , including terms in ωe ye v + 1 in Gv+1/2 , and estimate ωe ye . 2 8. The first strong electronic band system of carbon monoxide (the ground-state vibrational frequency of which is observed at 2140 cm−1 in the infrared) appears in absorption at room temperature at about 1550 Å in the vacuum ultraviolet. The system shows a progression with a spacing of 1480 cm−1 . The vibronic bands show a single set of unperturbed P–, Q–, and R–branches degraded to the red. Analysis by combination differences of these branches gives B� = 1.61 cm−1 , B�� = 1.93 cm−1 . In each band, the lines nearest the origin are P(2), e e Q(1), and R(0). (a) Deduce all you can about the two electronic states involved in the transition from these data and your general knowledge of the properties of carbon monoxide. (b) Sketch the lower portions of the potential curves in cm−1 for CO, roughly, to scale, from these data [find the harmonic force constant, and use the potential U (r) = 1 k(r − re )2 ]. Use the Franck-Condon principle 2 to find the strongest vibronic bands in the spectrum. (c) Interpret the electronic terms of the two states in terms of the most likely MO configurations of each. (d) The dipole moment of the ground state of CO is about 0.1 Debye (1 Debye = 10−18 esu-cm). Show how an optical Stark effect experiment can be used to find the dipole moment of the excited states; estimate the magnitudes of the splittings for the P(2), Q(1), and R(0) lines, for an applied field of 10,000 V/cm (1 statvolt = 300 ordinary volts), and an assumed excited-state moment of 1.0 Debye. 9. (a) What states arise from the σ2 σ∗, σ2 π, and σπ2 configurations? (There should be a total of 6 states.) (b) Write the linear combinations of Slater determinants which correspond to the e–parity components of each of these states. (There should be a total of 9 component states, hence 9 linear combinations.) (c) Which of these 9 substates can perturb each other? Make a table specifying the terms in the Hamiltonian � � HElect , HSO , HCoriolis which cause these perturbations. (d) Calculate all possible nonzero spin-orbit perturbation matrix elements between |ΛS Σ� |Ω JM |�e states in terms of the one-electron parameters � � a1 ≡ σ∗ |ξ�− |π a2 ≡ �σ|ξ�− |π� a3 ≡ �π|ξ�z |π� . (There will be at least 7 nonzero matrix elements.) 5.76 Problem Set #3 Spring, 1976 page 4 (e) In the absence of substate mixing, between which of these 9 e–parity substates can there exist electric dipole allowed R or P branch transitions? (f) Express the relative intensities of the following electric dipole transitions in terms of the one-electron � � matrix element µ⊥ ≡ σ|µ− |π : σ2 π 2 Π1/2 → σπ2 2 Σ+ σ2 π 2 Π3/2 → σπ2 2 Σ+ σ2 π 2 Π1/2 → σπ2 2 Σ− σ2 π 2 Π1/2 → σπ2 2 Δ3/2 σ2 π 2 Π3/2 → σπ2 2 Δ3/2 So there are some semi-empirical relationships among isoconfigurational transition intensities! 10. (a) Construct the Hamiltonian matrix for a 5 Δ state by evaluating the matrix elements of HROT = B(J−L−S)2 and HSO = AL · S in the Hunds case ‘a’ e/ f (parity) basis. (b) Construct the matrix of second-order Van Vleck Transformation corrections to HROT + HSO which results from the r–dependence of the “constants” Bv and Av . Define centrifugal distortion parameters Dv ≡ − � �v| B(r)|v� � �v� | B(r)|v� v� �v A Dv = 2 A◦ = v E v − E v� � �v|A(r)|v� � �v� | B(r)|v� E v − E v� v� �v � �v|A(r)|v� � �v� |A(r)|v� v� �v E v − E v� and express the matrix elements in terms of these three parameters. ...
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This note was uploaded on 11/28/2011 for the course CHEM 5.74 taught by Professor Robertfield during the Spring '04 term at MIT.

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