David R. Borst

Toluene: Structure, dynamics, and barrier to methyl group rotation in its electronically excited state. A route to IVR

Rotationally resolved fluorescence excitation spectra of several torsionally active bands in the S1–S0 electronic transitions of toluene and toluene-d3 have been recorded in the collision-free environment of a molecular beam. Analyses of these data provide accurate values of the internal rotor constants F; the barrier heights V6; the frame rotational constants AF; the overall rotational constants B and C; and the torsion-rotation coupling constants AF′; in the m=0 and m=±1 levels of the S0 state and the m=0, ±1, and 3+ levels of the S1 state. Comparison of the AF, B, and C values in the m=0 levels of the two states shows that S1 toluene is quinoidal in form, with shorter ring “parallel” C–C bonds than “perpendicular” ones, unlike the S0 state. The preferred conformation of the methyl group is staggered in both states, but the V6 values are significantly different; V6(S0)=−4.874 and V6(S1)=−26.376 cm−1. Comparison of the F, AF, and AF′ values in the different torsional levels of the S1 state shows that, be...

Hydrogen bonding and tunneling in the 2-pyridone·2-hydroxypyridine dimer. Effect of electronic excitation

Abstract The 2-pyridone·2-hydroxypyridine (2PY·2HP) mixed dimer has been studied using high resolution ultraviolet spectroscopy in the region of the 2PY S1–S0 origin, and fluorescence-dip infrared spectroscopy in the region of the hydride stretch fundamentals. The dense rotational structure of the electronic spectrum is characteristic of a b-type transition with a transition moment at 8°±3° to the b-axis, consistent with excitation of the 2PY half of the dimer. A tunneling splitting of 520±10 MHz appears in the spectrum, due to a double proton transfer in 2PY·2HP. The double proton transfer exchanges the chemical identity of the two monomer units, thereby leading to a double tautomerization. Theoretical calculations suggest that the barrier to such motion is about 8 kcal/mol in the ground state; hence, the observed tautomerization apparently occurs in the excited state. An approximate fit of the high resolution spectrum gives rotational constants that are consistent with an excited state structure in which only the OH⋯O hydrogen bond in the dimer is lengthened substantially. The infrared spectrum out of the pair of ground state zero-point tunneling levels in the XH stretch region is reminiscent of that in the pure (2PY)2 dimer. Its peak absorption frequency is at 2700 cm −1 , but the infrared band is spread over about 500 cm −1 , with reproducible sub-structure due to strong, anharmonic coupling. The excited state spectrum, in contrast, is dominated by a transition at 3135 cm −1 . This band is assigned to the OH fundamental, which is shifted to higher frequency by the weakening of the OH⋯O hydrogen bond upon electronic excitation.

On the additivity of bond dipole moments. Stark effect studies of the rotationally resolved electronic spectra of aniline, benzonitrile, and aminobenzonitrile

We study the influence of an applied electric field on the fully resolved electronic spectra of aniline (AN), benzonitrile (BN), and 4-aminobenzonitrile (ABN) in the gas phase. Using these data, we test the commonly held but rarely proven assumption that the total dipole moment of a polyatomic molecule is the vector sum of bond dipole moments, localized in different parts of the molecule. We find that μa(ABN)≈μa(AN)+μa(BN) in the excited S1 state, but not in the ground S0 state. Possible reasons for this non-additivity are discussed.

Styrene and phenylacetylene: Electronic effects of conjugating substituents “off” and “on” the axis of a benzene ring

Rotationally resolved fluorescence excitation spectra of several vibronic bands in the S1←S0 electronic transitions of styrene (STY) and phenylacetylene (PA) have been obtained. Confirming earlier low resolution results, we find that the origin band of PA is a b-type band but that the corresponding band of STY is an a-type band, showing that the S1 state of PA is 1Lb in character (like that of most other monosubstituted benzenes) but that the corresponding state of STY is 1La. The observed changes in the rotational constants of PA and STY that occur when the photon is absorbed are consistent with these assignments. Reversal in the electronic character of the S1 state in STY is attributed to the presence of the “off-axis” conjugating –CH=CH2 group, a suggestion that is supported by the observed polarizations of higher vibronic bands in both molecules.

High resolution electronic spectroscopy of three n-alkylbenzenes: ethyl-, propyl-, and butylbenzene

Rotationally resolved S1−S0 fluorescence excitation spectra of ethylbenzene, two conformers of n-propylbenzene, and two conformers of n-butylbenzene have been observed and assigned. The data obtained provide information about the equilibrium properties of each molecule, including their geometries in the S1−S0 states, their electronic distributions, and their dynamical behavior following the absorption of light. Trans structures are found to have S1 states that are 1Lb in character with relatively long fluorescence lifetimes. Gauche structures are found to have S1 states that are mixed (1Lb/1La) in character with relatively short fluorescence lifetimes. Possible reasons for these differences in properties are discussed.

Identification of the light-absorbing states in tolane with potential relevance to self-similar phenylacetylene dendrimers

Abstract We report the observation and analysis of the high resolution fluorescence excitation spectrum of tolane (or diphenylacetylene, DPA) in a molecular beam. This analysis shows that the first one-photon-allowed state in DPA has 1 B 1 u symmetry, a consequence of exciton-like coupling between the two 1 L a states of the connected phenylacetylene (PA) units. The resulting long-axis polarization of the 1 B 1 u – 1 A g transition of each linear segment is responsible for the diverse optical properties of PA dendrimers.

Molecular recognition in the gas phase. Dipole-bound complexes of benzonitrile with water, ammonia, methanol, acetonitrile, and benzonitrile itself

Described herein are the high resolution fluorescence excitation spectra of 1 : 1 complexes of benzonitrile with water, ammonia, methanol, acetonitrile, and benzonitrile itself in the gas phase. Analyses of these spectra yield the equilibrium geometries of each species in both the ground and excited electronic states, and therefore provide information about the intermolecular interactions that are responsible for holding them together, in the absence of perturbing solvent molecules. In all cases, the determined structure corresponds to the one expected on the basis of interacting dipole moments; in some cases, significant internal motions of one component relative to the other, leading to a time-varying dipole field, also has been observed.

Tunnelling splittings in the high resolution microwave and UV spectra of the benzonitrile—water complex: modelling the internal motion in its S0 and S1 states

Refined values of the barriers to internal motion in the 1:1 complex between benzonitrile and water in the gas phase have been determined from analyses of its fully resolved microwave and ultraviolet spectra. Both spectra exhibit tunnelling splittings associated with this motion. Modelling this behaviour using a semi-rigid Hamiltonian for an internal rotation of water around its C 2 axis yields values of V 2 = 440 ± 30 cm−1 and 450 ± 30 cm−1 in the ground and excited electronic states, respectively. These relatively high barriers are a consequence of two hydrogen bonds between the interacting species.