Trevor Ridley

The NH2-inversion potential function in the Ã1b2 electronic state of aniline: Evidence for planarity

Abstract Values for the NH 2 -inversion levels in the A 1 B 2 State of aniline-NH 2 and -ND 2 have been improved using new information regarding

The C(1)-C(α) torsional potential function in the ground electronic state of styrene obtained from the single vibronic level fluorescence and other spectra

Abstract The problem of the C(1)-C(α) torsional potential function of styrene has been solved using vibrational energy levels obtained from the single vibronic level fluorescence spectrum. The function is given by V = 1 2 ( V 2 cos2Φ + V 4 cos4Φ) where V 2 = 1145 ± 10 and V 4 = −278 ± 5 cm −1 . The V 4 , term is responsible for a very large amplitude torsional motion.

The 370-nm electronic spectrum of tropolone: Evidence from single vibronic level fluorescence spectra regarding the assignment of some vibrational fundamentals in the X̃ and à states

Abstract Single vibronic level fluorescence (SVLF) spectra of tropolone from vibronic levels in the A 1 B 2 electronic state, in combination with recently reported supersonic jet spectra, have enabled the assigning of many absorption bands in the region of 000 which had previously been impossible. Some of the complexity in these bands has been shown to be due to a large Duschinsky effect involving the two lowest b1 vibrations, ν25 and ν26. It has been shown that these vibrations have wavenumbers of 176 and 110 cm−1, respectively, in the X state, and 172 and 39 cm−1 in the A state. This last value shows how unresistent the molecule is in the A state to out-of-plane bending in the region of the five-membered ring. Other aspects of the vibrational complexity are due to the effect of ν′26 in increasing the barrier to tunnelling of the hydrogen-bonding proton in the A state contrasting with very little effect of ν″26 in the X state.

The Ã1A′-X̃1A′ single vibronic level fluorescence and Raman spectra of styrene-β-D2 vapor and their use in determining the C(1)-C(α) torsional potential function in the X̃ state

Abstract SVL fluorescence spectra of styrene-β-D2 (STYD2) are assigned and enable many new assignments to be made in the absorption spectrum. In turn these allow the determination of the first four levels of the C(1)-C(α) torsional vibration ν″42 in the ground electronic state. Using new information regarding the geometry of the molecule, in particular that 〉C(1)C(α)C(β) is about 130°, together with torsional vibrational levels from the electronic absorption spectrum and the Raman spectrum, has enabled us to determine the ν42″ torsional potential function V(φ) for styrene (STY) and STYD2: STY V(φ)/ cm −1 = [1070(1 − cos 2φ) − 275(1 − cos 4φ) + 7(1 − cos 6φ)] 2 ; STYD 2 V(φ)/ cm −1 = [1070(1 − cos 2φ) − 270(1 − cos 4φ) + 4.5(1 − cos 6φ)] 2 . This potential function changes appreciably when one quantum of ν″29, a substituent in-plane bending vibration, is excited. There is a very strong Duschinsky effect affecting ν42 and ν41, a substituent out-of-plane bending vibration, and intensity measurements in the SVL spectra have enabled us to obtain Duschinsky coefficients corresponding to the changes of normal coordinates from the X to the A state.

The Ã1A′-X̃1A′ single vibronic level fluorescence spectrum of styrene vapor

Abstract Single vibronic level (SVL) fluorescence spectra following excitation into the 0 0 0 band and many sequence and cross-sequence bands in the A 1 A′- X 1 A′ system of styrene have been recorded and assigned. The technique is shown to be very powerful in obtaining accurate energy levels for low-wavenumber vibrations, particularly in the X state, where it is capable of giving information regarding vibrational levels inaccessible by conventional infrared or Raman spectroscopy. Interpretation of the spectra leads to many new assignments in the absorption spectrum and to an accurate knowledge of many vibrational energy levels. An important reassignment is that of the 41 0 1 42 0 1 band previously assigned to 40 0 2 ( ν 40 , ν 41 , and ν 42 are all a ″ vibrations). The two most important pieces of information which derive from analysis of the SVL spectra are (a) the first five vibrational levels of ν 42 , the C (1)- C ( α ) torsional vibration, in the X state, each accurate to about ±0.2 cm −1 , and (b) the fact that the normal coordinates of ν 42 and ν 41 , an out-of-plane substituent vibration, are very heavily mixed in the A , relative to the X , state—an extreme case of the Duschinsky effect. As a result of analysis of the SVL spectra, analysis of the absorption spectrum is now so complete that we can be confident that the Hui and Rice proposal that the CH 2 group of the substituent is perpendicular to the plane of the rest of the molecule in the A state has no evidence to support it.

The A˜1A′−X˜1A′ absorption and single vibronic level fluorescence spectra of 2-aminopyridine

Abstract It has been shown previously [ J. M. Hollas, G. H. Kirby, and R. A. Wright, Mol. Phys.18, 327–335 (1970) ] that there is a close similarity between the rotational contours, and almost certainly the geometry changes, in the A ˜ − X ˜ systems of 2-aminopyridine and aniline. Here it is shown, by a combination of absorption and SVL fluorescence spectra, that the similarity extends to the activity of ν1, the NH2-inversion vibration, in the A and X ˜ states. The I11, I02, and I20 bands are intense and their assignments have been confirmed. It seems certain that there is a large decrease in the out-of-plane angle of the hydrogen atoms of the NH2 group from the X ˜ to the A state perhaps, like aniline, resulting in planarity in the A state. A very large Duschinsky effect involving the a″ vibrations ν29 and ν31 enables the identification of these two fundamentals, in the X ˜ state, in the gas-phase far-infrared spectrum [ R. A. Kydd, Spectrochim. Acta A35, 409–413 (1979) ]. Other information from SVL fluorescence spectra also leads to the identification of ν″22, ν″23 and ν″30 from the far-infrared spectrum. The intensity distribution along the 22n1 progression shows a minimum at 2211. Franck-Condon factor calculations of this intensity pattern show that the change in Q22(Q6a) from the X ˜ to the A state is 0.343u1/2 A. A comparison between photographic and photoelectric recording of the 000 and I11 SVL fluorescence spectra shows that photographic recording is slower by a factor of at least 5. The only virtue of photographic recording is the ease of calibration.

The Ã1A1-X̃1A1 two-photon fluorescence excitation spectrum of 1,2-difluorobenzene

Abstract The two-photon fluorescence excitation spectrum of 1,2-difluorobenzene was obtained with a tunable dye laser calibrated using a combination of the neon optogalvanic spectrum and etalon fringes. The spectrum consists only of A1-A1 bands but the use of linear and circular polarization divides the bands into two types. The 000 type retains its intensity in circular polarization and, rotationally, shows little or no zero-rank contribution. The 510 (or 1410) type loses much of its intensity in circular polarization and, rotationally, shows a large zero-rank contribution. These observations all accord with the trace of the two-photon transition tensor being close to zero for the 000 type and large for the 510 type, the latter type being involved in vibronic interaction which mixes the A and X states. There is strong evidence for Fermi resonance between the 51 and 61101 levels. Parts of the one-photon absorption spectrum have been photographed to aid sequence identification and also to look for the 510, A1-A1 transition. This transition is not observed: nor is there any evidence for intensity stealing by b2 vibrations.

The Ã1B2-X̃1A1 two-photon fluorescence excitation spectrum of 1,3-difluorobenzene

Abstract The A 1 B 2 - X 1 A 1 system of 1,3-difluorobenzene has been observed using the technique of two-photon fluorescence excitation obtained with a pulsed dye laser. Calibration was achieved by a combination of the neon optogalvanic spectrum and etalon fringes. In circular, compared to linear, polarization the bands divide into two groups, those which are B2-A1 and which retain their intensity with circular polarization, and those which are A1-A1 and lose about 60% of their intensity under the same conditions. These two kinds of bands also show characteristic rotational contours. All of the A1-A1 bands whose assignments are established obtain their intensity through vibronic interaction in which the vibration ν′25 (ν′14 in the Wilson numbering) mixes the A with, presumably, the X state. There is an important Fermi resonance between the 91 and 101111 levels. Parts of the one-photon absorption spectrum have been photographed to identify sequences associated with the 000 band for comparison with those observed in the two-photon spectrum, and to search for bands involving odd quanta of b2 vibrations, including ν′25 (ν′14); none was found.

The A˜1A′−X˜1A′ absorption and single vibronic level fluorescence spectra of some deuterated 2-aminopyridines

Abstract The A ˜ − X ˜ systems of three 2-aminopyridines, fully or partially deuterated on the NH2 group, have been investigated by means of absorption and SVL fluorescence spectra. Absorption bands of a mixture of four species (three deuterated and one undeuterated) can be attributed to a particular species primarily by observing transitions involving the deuterium-sensitive NH2-inversion vibration in the corresponding SVL fluorescence spectra. The dependence of the wave-number of the vibration ν22 on the degree of deuteration is also helpful in this respect. Evidence for appreciable hydrogen bonding between the ring nitrogen and the hydrogen atom of the NH2 group is strongly supported by the resulting spectra. The inversion vibration levels for the two partially deuterated species are very different and there is evidence from the spectrum of 2-aminopyridine (-ND2) that, what would be purely torsional motion of an NH2 group free from hydrogen bonding, is appreciably modified in 2-aminopyridine.

The 330-nm S1-S0 electronic spectrum of 1-pyrazoline: Probable hindered pseudorotation in S1 and confirmation of the ring-puckering potential function in S0 by single vibronic level fluorescence spectroscopy

Abstract The S1-S0 absorption spectrum of 1-pyrazoline is rotationally sharp but vibrationally extremely irregular, and other techniques are necessary to aid its assignment. The relaxed fluorescence spectrum shows a very long progression in the Nue5fbN twisting vibration, suggesting that the ring is twisted in S1 whereas, in S0, this part of the ring is planar but the CH2 group in position 4 is puckered. With a twisted ring in S1 it seems likely that the Nue5fbN twisting and CH2(4) puckering modes in S0 should be combined and newly described as radial and hindered pseudorotational modes in S1. The vibronic transitions accompanying such an S1-S0 electronic transition are derived. Single vibronic level fluorescence spectra from many vibronic levels of S1 show progressions in both the Nue5fbN twisting and CH2(4) puckering vibrations in S0, but only with Δv even. This strongly supports the suggestion that these two modes are heavily mixed in S1, and indicates that the fluorescing states are either above the barrier to pseudorotation or not far below it, so that there is appreciable tunnelling through the barrier. The progressions in the CH2(4) puckering vibration allow us to assign uniquely the puckering quantum number, in S0, of the band in which excitation took place. In addition, the spacings in these progressions further confirm the preferred potential function derived from the far-infrared spectrum and confirmed previously from the microwave spectrum.