J.C.D. Brand

The vibrational spectrum and torsion of phenol

Abstract The complete vibrational assignment is presented for phenol- d 5 , and previous assignments for the normal and - d 1 isotopes are slightly amended. The substituent couples the a 1 and b 2 displacements of the phenyl ring in such a way that the in-plane fundamentals all appear in the spectrum as essentially A -type bands. The torsional infrared frequencies, together with the subtorsional splittings derived from microwaves, lead to an estimate of the twofold barrier hindering internal rotation, V 2 = 1215 cm −1 . Inconsistencies between the various isotopes indicate that the uncertainty in V 2 may be about 50 cm −1 . The moment of inertia for internal rotation of the hydroxyl group shows that the oxygen and hydrogen nuclei lie on opposite sides of the torsional axis, the CO bond being inclined to the axis at about 5°. The off-axis displacement of the oxygen nucleus is calculated to be ∼0.12 A.

Vibrational analysis of the first ultraviolet band system of aniline

Abstract Vibrational structure associated with the 2940-A band system of aniline vapor has been analyzed in some detail, and results are included for the − d 2 , − d 5 , and − d 7 isotopes. It is shown that the ω(NH 2 ) vibration is strongly anharmonic in the ground state, having the small-large alternation of quanta characteristic of a vibration occurring in a double-minimum potential. The data are consistent with an angle of about 46° between the ring-to-N bond and the NH 2 plane in the electronic ground state. The same vibration is not harmonic in the upper electronic state, though its anharmonic character is less strongly marked there than in the ground state. The data for the upper state are compatible with a double-minimum potential having a very small maximum for the all-coplanar configuration, but the evidence is not conclusive. All that can be said definitely is that ∂ 3 V ( q )∂ q 3 ≠ 0 for a considerable range of q to either side of q = 0. The term quasiplanar is proposed to cover this situation, a few other examples of which can be found in the literature. A complete classification of the vibronic sublevels of aniline is accomplished using a group G 8 of order eight; but the torsional splittings are not resolved in the electronic spectrum so that the practically useful group is G 4 , a sub-group of G 8 . G 4 is isomorphous with the point group C 2 v . The vibronic selection rules are discussed.

The 2750-Å band system of phenol: Part II. Extended vibrational assignments and band contour analysis

Abstract Vibrational analysis of the electronic bands of phenol vapor at 2500–2900 A has been extended to the a1 modes of phenol-d5 and the a2, b1, and b2 modes of phenol-h6, -d1, and -d5. The results give an essentially complete set of frequencies for the fundamentals lying below 2000 cm−1 in the excited state. Ground-state fundamentals to about 1000 cm−1 can be identified from “hot” bands in the electronic spectrum and are generally in good agreement with the infrared vapor absorption. The envelopes of A-, B-, and C-type bands have been calculated in the rigid rotor approximation for a range of values of the excited-state inertial constants. Features in the origin band contour are matched in the computed envelope when, and only when, the band is taken as B-type (y-polarized); therefore the electronic transition is B2 ← A1, analogous to the B2u ← A1g transition of benzene. The changes in rotational constants accompanying the transition are ΔA = −0.01135 and ΔB = 0.00116 cm−1. Combined with micro-wave data for the ground state, these constants lead to approximate values for Ia and Ib in the electronically excited state. While the results are not sufficient for a structure determination, they show that the phenyl ring is almost certainly not a regular hexagon in the B2 state, though the question whether the distortion is associated with unequal CC bond distances (“quininoid” structure) or unequal angles within the ring, or from a blend of both, cannot be decided. The picture that emerges is consistent qualitatively with the fact that torsional motion in the B2 state is much more rigid than in the ground state, the torsional barrier being about 3.5 times greater. It therefore appears probable that the phenyl ring has appreciable quininoid character in the B2 state.

The 2750-Å electronic band system of phenol: Part I. The in-plane vibrational spectrum

Abstract The banded absorption of phenol vapor in the region 2500–2900 A has been extensively analyzed and the vibrational structure associated with the in-plane modes has been interpreted. Vibronic selection rules and the symmetry classification of states is discussed in the framework of a molecular symmetry group G 4 , isomorphous with the point group C 2 v . The spectrum comprises a strong, allowed system of bands polarized in the plane of the molecule perpendicular to an axis grazing the O-atom (“short axis”), and a group of much weaker, forbidden subsystems polarized along this axis (“long axis”). All a 1 vibration frequencies of the phenyl group in the excited state of phenol have been assigned and measured. The vibrations principally active in the forbidden subsystems are 6 b ( b 2 ring deformation), 9 b ( b 2 CH bending) and 7 b ( b 2 CH stretching), all of which correlate with e 2 g modes of benzene. The forbidden subsystems collectively have similar intensity (10 4 f = 8) to the corresponding 2600-A bands of benzene (10 4 f = 14), though the phenol mode 6 b is relatively less effective in intensity borrowing than mode 6 of benzene. Intensity distribution in the a 1 progressions is consistent qualitatively with a geometry change on excitation encompassing ( i ) a generalized increase in CC bond distance, ( ii ) a decrease in CO distance, and ( iii ) some increase in quininoid character in the aromatic ring. An a priori calculation of allowed and forbidden intensities is attempted, using the Herzberg-Teller theory with the inclusion of higher terms. It is shown that terms in the conventional Herzberg-Teller expansion contribute alternately to the intensity of the allowed and forbidden systems , so that only terms 1, 3, 5, ⋯ give rise to nonzero values in an allowed subsystem, while only terms 2, 4, ⋯ have nonzero values in a forbidden subsystem. However, quantitative results are not satisfactory, probably owing to inaccuracies in the electronic wave functions.

The 4750 Å band system of chlorine dioxide

Abstract The 4750 A band system of ClO 2 was examined under high resolution using 35 Cl and 37 Cl isotopes. A revised vibrational analysis leads to new molecular constants for the ground and excited states, the electronic term value and harmonic frequencies of 35 ClO 2 being: T ′ 000 = 21016.3, ω ″ 1 = 963.5, ω ″ 2 = 451.7, ω ″ 3 = 1133.0, ω ′ 1 = 722.4, ω ′ 2 = 296.3 and ω ′ 3 = 780.1, all in cm −1 . In agreement with earlier analyses, the 2-0 and 4-0 transitions in ν 3 are present in the spectrum with intensities comparable to that of the origin band. This observation is not compatible with the expectation based on C 2 v structures when the vibrational wave function is expressed as a simple product of harmonic oscillator functions , and has previously been put forward as evidence for a C s excited state structure in which the equilibrium Clue5f8O distances are unequal. The isotopic data now available are not consistent with the existing model of this unsymmetrical structure: instead, the probable explanation of the ν 3 “progression” is that anharmonic terms in the linear-corrected vibrational wave function, especially terms associated with the cubic force constant k 133 , allow these transitions to steal intensity from the principal ν 1 progression, the physical structure being symmetrical ( C 2 v ) in both electronic states.

The 4750-Å band system of chlorine dioxide. Rotational analysis, force field and intensity calculations

Abstract The structure of the ν1 vibronic band in the 2 A 2 ← 2 B 1 electronic band system of chlorine dioxide has been rotationally analyzed as an A-type (parallel) band of a near-prolate asymmetric rotor. Excited state constants ν0, A, B, C, τaaaa, τbbbb, τaabb, and τabab were obtained by a least-squares term value analysis based on about 730 assignments in the (100) ← (000) band, the results for 35ClO2 being: ν 0 21 727.087±0.020 cm −1 τ aaaa (−0.1529±0.0010)×10 −3 cm −1 , A 100 1.05633±0.00013 cm −1 τ bbbb (−0.0077±0.0003)×10 −3 cm −1 , B 100 0.30950±0.00006 cm −1 τ aabb (0.0126±0.0015)×10 −3 cm −1 , C 100 0.23815±0.00006 cm −1 τ abab (−0.0026±0.0007)×10 −3 cm −1 , The rotational analysis did not however distinguish between alternative sets of spin-rotation coupling constants, nor could a firm decision be reached from structural considerations. Quadratic, cubic, and quartic constants of the excited state force field have been evaluated using a model potential function to restrict the number of independent terms in the cubic and quartic portions of the field. The cubic constants are used to calculate “equilibrium” values of the excited state rotational constants from those observed for the (100) vibrational state, leading to a set of “equilibrium” moments of inertia, 35 ClO 2 :I a =15.48, I b =54.16, I c =69.63 amu A 2 , and hence to an excited state structure, r( ClO )=1.619 A , ∠ OClO =107°0′ . Even-quantum changes (2-0 and 4-0 transitions) in the b2 mode ν3 are relatively prominent in the vibrational structure of the band system. In order to test the possibility that these bands “borrow” their intensity from transitions involving the totally-symmetrical modes (mainly, from bands of the ν1 progression), relative intensities were calculated using second-order corrected anharmonic vibrational wavefunctions in which the harmonic frequencies and higher-order potential constants appear as coefficients of harmonic oscillator functions. These calculations give a good account of the observed intensities. Some further features, not readily explained by this approach, are noted.

Aniline-planar or nonplanar?

Abstract The question whether the NH 2 group of aniline is coplanar with the aromatic ring has been raised many times ( 1 ) but the configuration has not yet been established beyond doubt. Evidence from the electronic band system near 2940 A appears to settle this point, not only for the ground state of aniline but for its first electronically-excited state ( 1 B 2 ) also.

The 2750 Å electronic band system of phenol: Calculation of out-of-plane frequencies and sequence intensities

Abstract Force constant calculations are reported for the out-of-plane vibrational modes of phenol in its electronic ground state and first singlet excited state at 36349 cm −1 , using a limited potential energy matrix of 22 nonzero elements. These calculations, which are exploratory in nature, were undertaken in an attempt to evaluate three questions of general interest, namely, (i) Can the intensity of the Δ v = 0 transitions (sequences) in an electronic spectrum be harnessed to check the assignment of nontotally symmetrical vibrations? (ii) Does the fact that the out-of-plane vibration frequencies of phenol in its electronic ground state are essentially the same as those of fluorobenzene indicate that the OH torsion is only loosely coupled to the phenyl modes? (iii) Is a C 2 v classification of practical value for the out-of-plane modes of phenol in either electronic state? It is found that (i) and (ii) can be answered affirmatively, with slight reservations. The C 2 v species are of some value in classifying displacements in the phenyl group, but may be misleading if applied indiscriminately.