J.Michael Hollas

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.

Single-vibronic–level fluorescence spectra of aniline and aniline–argon clusters in a supersonic jet

A supersonic-jet apparatus has been constructed employing a continuous nozzle and large pumping capacity. A set of concave mirrors in a ‘Welsh’ arrangement is used inside the vacuum chamber for efficient collection of the fluorescence. The fluorescence excitation spectrum of the 000 band of the A–X system of aniline (AN) shows a characteristic rotational contour, and computer simulation, with type-B rotational selection rules, shows that the rotational temperature is ca. 10 K. A type-A contour computed with similar parameters shows that low-resolution type-A and -B contours may not be easy to distinguish and at low temperatures. The single-vibronic-level fluorescence (SVLF) spectrum of AN with 000 excitation shows the 6b01 band very weakly. Vibration ν6b is of b2 symmetry, and this type of vibronic coupling, which is much stronger in, for example, toluene, fluorobenzene and phenol, has been demonstrated in AN for the first time. SVLF spectra following excitation into the 16a20 and 10b20 bands, which are 6 cm–1 apart, confirms their assignments. Since ν16b and ν10b are of different symmetry, a2 and b1, respectively, and they have a similar wavenumber, there is a Darling–Dennison resonance between the 16a2 and 10b2 vibronic levels. SVLF spectra obtained by tuning the laser to the 000 band of the van der Waals complex AN–Ar1 show that the fluorescence occurs not only from the 00 level of AN–Ar1 but also from the 00 level of bare AN. It is suggested that the bare AN fluorescence originates from the pseudo-continuum which underlies the 000 band of AN–Ar1 and is due to larger AN–Arn clusters, which may be unstable with respect to Ar bending motions in 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.

Supersonic jet fluorescence and gas phase absorption spectra of Indan : puckering potential in S0 and S1

Abstract Single vibronic level (dispersed) fluorescence spectra of indan in a supersonic jet have been obtained and interpreted. The observation of transitions in the puckering vibration ν 27 of the five-membered ring with Δ v 27 even and odd confirm that the equilibrium conformation in S 0 is puckered in a C s configuration. The barrier to planarity of the carbon skeleton is 1979 cm −1 . These spectra force a reinterpretation of some bands in the gas phase electronic absorption spectrum in which the 27 0 1 band is now seen to be prominent. The observation of this and other 27 v v+1 bands, which show splittings due to tunneling through the puckering barrier, indicate a lower barrier of about 1800 cm −1 in S 1 but the v ′ 27 = 0, 1, and 2 levels cannot be fitted very well to a quadratic-quartic potential. A probable reason for this is that the vibrational motion involved in ν 27 in S 1 is no longer a purely puckering motion.

The C(1)-C(α) torsional potential function of 2-fluorostyrene from S1-S0 fluorescence in a supersonic jet

Abstract Single vibronic level fluorescence spectra of trans-2-fluorostyrene have been observed in a supersonic jet. Levels of the C(1)-C(α) torsional vibration up to ν=8 are fitted to a potential function which shows that the molecule is quasi-planar with a barrier of 15±4 cm−1.

Supersonic jet fluorescence spectroscopy of 1,3-benzodioxole: A non-planar (C2) structure in S0

Abstract 1,3-benzodioxole has previously been shown to be planar, with respect to CH 2 puckering, in the S 0 state. We have shown that it is also planar, in this respect, in the S 1 state. However, jet fluorescence spectra show that the oxygen atoms are twisted out-of-planein S 0 .

Evidence for two rotational isomers of 1-naphthol and 2-naphthol from their gas-phase electronic absorption spectra

Abstract In a previously reported S 1 - S 0 fluorescence excitation spectrum of 2-naphthol in a supersonic jet there is strong evidence for cis and trans rotamers with their 0 0 0 bands separated by 318 cm −1 . The high-resolution gas-phase absorption spectrum has been reinvestigated and produces confirmatory evidence. It shows that some sequence band intervals associated with the two 0 0 0 bands differ by more than would be expected if both bands were due to the same rotamer. Also, deuteration of the OH group results in an increase in the separation of the 0 0 0 bands which, together with evidence from the supersonic jet spectrum, is incompatible with their belonging to the same rotamer. The absorption spectrum of 1-naphthol shows very similar behavior and suggests that this molecule also exists as cis and trans rotamers. In both 1- and 2-naphthol the more stable rotamer in S 0 becomes the less stable rotamer in S 1 . Steric factors seem to be unimportant in determining which is the more stable rotamer.