What is the best DFT functional for vibronic calculations? A comparison of the calculated vibronic structure of the S1-S0 transition of phenylacetylene with cavity ringdown band intensities.

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.

An ab initio molecular orbital study of intramolecular hydrogen bonding in ortho-substituted arylamides: implications for the parameterization of molecular mechanics force fields.

The aromatic oligoamide (arylamide) foldamer class, characterized by the repetitive aromatic-amide pattern, is one of the most intensively studied foldamer families. In this article, the potential energy profiles with regard to torsional motions around the two types of aromatic-amide bonds (C(a)-C(p) and C(a)-N) are obtained at the B3LYP/6-311G(d,p) level of theory. The effect of ortho substituents with different hydrogen bonding abilities (OCH(3) vs. SCH(3) ) on the torsional potential profiles is analyzed in detail. There are several findings that have implications in foldamer design. The ortho-SCH(3) substituent on the benzene ring produces a much more flexible arylamide backbone with respect to the OCH(3) substituent, as it restricts the C(a)-C(p) torsion to a lesser extent. Interestingly, the rigidifying effect of the ortho-SCH(3) substituent on the C(a)-N torsion is very similar to that of the OCH(3) substituent on the same linkage type. In addition, the SCH(3) substituent prefers a perpendicular orientation with respect to the benzene ring to the in-plane one. It is also found that reparameterization of the corresponding torsional parameters, sometimes specific to the ortho substituent type, in the general amber force field is necessary for an accurate description of the backbone torsions in arylamides. Six sets of partial charge/torsional parameters for each linkage (C(a)-C(p) or C(a)-N)/substituent (OCH(3) or SCH(3) ) combination are obtained based on the ab initio torsional profiles. Initial assessments of these parameters show good agreement with the ab initio results.

State-to-state reaction dynamics: a selective review.

A selective review of state-to-state reaction dynamics experiments is presented. The review focuses on three classes of reactions that exemplify the rich history and illustrate the current state of the art in such work. These three reactions are (1) the hydrogen exchange reaction, H+H2-->H2+H and its isotopomers; (2) the H+RH-->H2+R reactions, where RH is an alkane, beginning with H+CH4-->H2+CH3 and extending to much larger alkanes; and (3) the Cl+RH-->HCl+R reactions, principally Cl+CH4-->HCl+CH3. We describe the experiments, discuss their results, present comparisons with theory, and introduce heuristic models.

Experimental and theoretical study of the electronic spectrum of the methylene amidogen radical (H2CN): verification of the 2A1 <-- 2B2 assignment.

A collaborative experimental and theoretical study of the electronic spectrum and excited-state photochemistry of H(2)CN has been carried out. The absorption spectrum, in the range of 287-278 nm, was measured through cavity ring-down spectroscopy. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features were observed in the 34800-35800 cm(-1) spectral range, along with the A-X (1,0) band of the OH cofragment. The broad features were assigned through high-level ab initio calculations as vibronic transitions to the ground and 2b(1) (umbrella mode) levels of the second excited B (2)A(1) state from the ground X (2)B(2) state of H(2)CN. Rotational constants for the lower and upper levels of these transitions were computed from the expectation values of the moments of inertia tensor, using the appropriate vibrational wave functions. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric dipole-allowed (2)A(1) <-- (2)B(2) transition appropriate to this transition. The former assignment to the dipole-forbidden (2)B(1) <-- (2)B(2) transition can be ruled out by these results. A theoretical investigation of the dissociation pathways for electronically excited H(2)CN is also presented. The upper states of the observed bands cannot dissociate directly but rather decay through internal conversion and subsequent dissociation to H + HCN fragments; higher b(1) levels are above the excited-state dissociation limit.