David W. Pratt

Structure and vibrational dynamics of aniline and aniline–Ar from high resolution electronic spectroscopy in the gas phase

Rotationally resolved S1←S0 electronic spectra of aniline and its single atom van der Waals complex with argon (An–Ar) have been observed. Analysis of these spectra leads to a determination of the vibrationally averaged structures of the bare molecule and the complex in the two electronic states. Aniline itself is pyramidally distorted at the NH2 group in the S0 state. Attachment of the Ar atom on the side of the ring opposite the two N–H bonds converts the symmetric double well along the inversion coordinate into an asymmetric one, in the ground state. The excited state is quasiplanar along this coordinate. Analyses of the spectra of An–Ar at higher energies in the S1 state provide a probe of the vibrational predissociation (VP) behavior of the complex. We observe in these spectra line broadenings and spectral perturbations from which the important role of intra–intermolecular mode mixing (i.e., IVR) in promoting the VP process is elucidated.

High resolution S1←S0 fluoescence excitation spectra of the 1‐ and 2‐hydroxynaphthalenes. Distinguishing the cis and trans rotamers

Both fluorescence excitation and dispersed emission techniques have been used to study the S1←S0 electronic spectra of 1‐ and 2‐hydroxynaphthalene (1/2HN) in the collision‐free environments of a supersonic jet and a twice‐skimmed molecular beam, using both pulsed and high‐resolution cw lasers operating in the ultraviolet. In the jet experiments, we observe that each molecule exhibits two electronic origins, separated by 274 cm−1 in 1HN and by 317 cm−1 in 2HN. In the beam experiments, we resolve the rotational structure of each of the four bands and determine the inertial constants of all eight zero‐point vibrational levels, accurate to ±0.1 MHz. We also determine the orientations of the four optical transition moments in the molecular frame. Significant differences in both the inertial constants and the transition moment orientations are observed in each band. Similar experiments have been performed on the hydroxy‐deuterated 1/2HN (1/2DN).A comparison of the results obtained for 1/2DN with those for the c...

Inertial axis reorientation in the S1←S0 electronic transition of 2‐pyridone. A rotational Duschinsky effect. Structural and dynamical consequences

Rotationally resolved fluorescence excitation spectra of two vibronic bands in the S1←S0 electronic transition of 2‐hydroxypyridine (2HP), and of the corresponding bands in the hydroxy‐deuterated molecule, have been obtained. A comparison of the rotational constants of the two molecules shows that the two bands both originate in the zero‐point vibrational level of the planar keto tautomer of 2HP, 2‐pyridone (2PY), and terminate in different zero‐point levels of 2PY that have different out‐of‐plane equilibrium geometries at nitrogen. Additionally, all four bands exhibit ‘‘anomalous’’ rotational line intensities that are shown to result from an in‐plane inertial axis reorientation which occurs on absorption of the photon. Likely atomic displacements that are responsible for this ‘‘rotational’’ Duschinsky effect, which may have significant dynamical consequences in 2PY and other molecules, are discussed.

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...

Intersystem crossing in isolated molecules. Magnetic field effects on the fluorescence decay behavior of 1B3u pyrazine with ‘‘single’’ rovibronic level excitation

Coherently prepared ‘‘single’’ rovibronic levels of electronically excited pyrazine typically exhibit a biexponential fluorescence decay whose preexponential factor ratio A+/A− is magnetic field dependent. Studies of this dependence for different J′ values show that complete spin decoupling from the molecular frame, leading to the intersystem crossing (ISC) selection rule ΔJ=0,±1, occurs at the anomalously low field of ∼100 G. The monoexponential decay of the J′=0 level is independent of the field. An analysis of these results suggests that Coriolis coupling plays an important role in the ISC process, a fact which explains the magnetic quenching behavior of many polyatomic molecules.

Methyl group torsional dynamics from rotationally resolved electronic spectra. 1‐ and 2‐methylnaphthalene

Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S1←S0 transitions of 1‐ and 2‐methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion–rotation perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V3 (the threefold torsional barrier) decreases from 809 cm−1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first‐order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of ...

Laser‐induced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetyl

We report the first structural study of the lowest triplet states of three α‐dicarbonyls (glyoxal, methylglyoxal, and biacetyl) using the technique of laser‐induced phosphorescence (LIP) spectroscopy in supersonic jets. At the level of vibrational resolution, 3Au glyoxal appears to have a geometry very similar to that of the ground state. But the T1←S0 transitions of methylglyoxal and biacetyl each exhibit strong progressions in the torsional vibrations of the methyl groups, showing that these molecules undergo a conformational change on excitation to the lowest triplet state. A Franck–Condon analysis of the methylglyoxal spectrum, with proper consideration for nuclear spin statistics, yields a methyl barrier of V3=115±5 cm−1 in this state. This value has been confirmed by a direct measurement of the tunneling splitting of A and E torsional levels. The hindering potential in the lowest triplet state of methylglyoxal is substantially different from those in the ground (V3=269 cm−1) and first excited single...

Vibronic coupling in indole: I. Theoretical description of the ¹La-¹Lb interaction and the electronic spectrum

The properties of the three lowest singlet electronic states (ground, (1)L(b), and (1)L(a) states) of indole (C(8)H(7)N) have been calculated with second-order approximate coupled-cluster theory (CC2) within the resolution-of-the-identity approximation. Refined electronic energies at the CC2 optimized structures and transition dipole moments were calculated using a density functional theory multi-reference configuration-interaction (DFT/MRCI) approach. Structures, energies, and dipole moments are reported for all three states and compared to experimental values. From the optimized structures and calculated transition dipole moments, we predict that pure (1)L(b) bands will have positive signs for both the axis reorientation angle theta(T) and the angle theta of the transition dipole moment with respect to the inertial a axis. For (1)L(a) bands the signs of both angles will be reversed. Vibronically coupled bands can exhibit opposite signs for theta and theta(T). The absorption and emission spectra of indole are calculated based on the Franck-Condon Herzberg-Teller approximation using numerical transition dipole moment derivatives at the DFT/MRCI level of theory. Implications for the experimentally observed vibronic spectra are discussed. Predictions are made for rotationally resolved spectra of various rovibronic bands. A conical intersection, connecting the (1)L(b) and (1)L(a) states, which can be accessed to varying extents via different Herzberg-Teller active modes is found approximately 2000 cm(-1) above the (1)L(b) minimum.

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.

Hydrogen bonding in the symmetry‐equivalent C2h dimer of 2‐pyridone in its S0 and S2 electronic states. Effect of deuterium substitution

The properties of the two intermolecular N–H⋅⋅⋅O bonds that are responsible for the formation of the cis‐peptide‐like dimer of 2‐pyridone (2PY) have been examined using deuterium substitution of the bridging hydrogen atoms as a probe. Studies of the fully resolved S2←S0 electronic spectrum of (2PY)2 in a molecular beam show that the protonated dimer has a symmetry‐equivalent planar C2h structure in its S0(1Ag) and S2(1Bu) states. Analogous studies of (2PY)2–d1 and (2PY)2–d2 show that (2PY)2 and (2PY)2–d2 are energy delocalized dimers in their S2 states, with an exciton splitting of less than 20 cm−1. Effective structures of the symmetric dimers in both states are derived from the measured rotational constants. A comparison of these structures shows that there is a distortion of the hydrogen‐bonding geometry when hydrogen is replaced by deuterium, along both in‐plane and out‐of‐plane coordinates. ΔR(N–H⋅⋅⋅O)=0.008 A, Δθ[C=O⋅⋅⋅(H)–N]=0°, and Δφ (the dihedral angle)=0.96° in S0 (2PY)2–d2 and ΔR=0.003 A, Δθ=0...