Infrared overtone spectroscopy and vibrational analysis of a Fermi resonance in nitric acid: Experiment and theory

High resolution infrared spectra of nitric acid have been recorded in the first OH overtone region under jet-cooled conditions using a sequential IR-UV excitation method. Vibrational bands observed at 6933.39(3), 6938.75(4), and 6951.985(3) cm(-1) (origins) with relative intensities of 0.42(1), 0.38(1), and 0.20(1) are attributed to strongly mixed states involved in a Fermi resonance. A vibrational deperturbation analysis suggests that the optically bright OH overtone stretch (2nu1) at 6939.2(1) cm(-1) is coupled directly to the nu1 + 2nu2 state at 6946.4(1) cm(-1) and indirectly to the 3nu2 + nu3 + nu7 state at 6938.5(1) cm(-1). Both the identity of the zero-order states and the indirect coupling scheme are deduced from complementary CCSD(T) calculations in conjunction with second-order vibrational perturbation theory. The deperturbation analysis also yields the experimental coupling between 2nu1 and nu1 + 2nu2 of -6.9(1) cm(-1), and that between the two dark states of +5.0(1) cm(-1). The calculated vibrational energies and couplings are in near quantitative agreement with experimentally derived values except for a predicted twofold stronger coupling of 2nu1 to nu1 + 2nu2. Weaker coupling of the strongly mixed states to a dense background of vibrational states via intramolecular vibrational energy redistribution is evident from the experimental linewidths of 0.08 and 0.25 cm(-1) for the higher energy and two overlapping lower energy bands, respectively. A comprehensive rotational analysis of the higher energy band yields spectroscopic parameters and the direction of the OH overtone transition dipole moment.     

Spectroscopic observation of a group 12 oxyfluoride: a matrix-isolation and quantum-chemical investigation of mercury oxyfluorides

Getting to the root of Hg: Experiments with amalgams as a source for laser-ablated Hg atoms as reaction partners with OF(2) gave strong HgF(2) IR absorptions and new bands in the Hg?F stretching region for OHgF and FOHgF molecules trapped in solid neon and argon. Assignment of these new bands to the first oxyfluoride of mercury, OHgF, and to the FOHgF insertion product is supported by quantum-chemical methods.     

Functionally Important Aromatic-Aromatic and Sulfur-π Interactions in the D2 Dopamine Receptor

The recently published crystal structure of the D3 dopamine receptor shows a tightly packed region of aromatic residues on helices 5 and 6 in the space bridging the binding site and what is thought to be the origin of intracellular helical motion. This highly conserved region also makes contacts with residues on helix 3, and here we use double mutant cycle analysis and unnatural amino acid mutagenesis to probe the functional role of several residues in this region of the closely related D2 dopamine receptor. Of the eight mutant pairs examined, all show significant functional coupling (Ω > 2), with the largest coupling coefficients observed between residues on different helices, C3.36/W6.48, T3.37/S5.46, and F5.47/F6.52. Additionally, three aromatic residues examined, F5.47, Y5.48, and F5.51, show consistent trends upon progressive fluorination of the aromatic side chain. These trends are indicative of a functionally important electrostatic interaction with the face of the aromatic residue examined, which is likely attributed to aromatic-aromatic interactions between residues in this microdomain. We also propose that the previously determined fluorination trend at W6.48 is likely due to a sulfur-π interaction with the side chain of C3.36. We conclude that these residues form a tightly packed structural microdomain that connects helices 3, 5, and 6, thus forming a barrier that prevents dopamine from binding further toward the intracellular surface. Upon activation, these residues likely do not change their relative conformation, but rather act to translate agonist binding at the extracellular surface into the large intracellular movements that characterize receptor activation.     

ChemInform Abstract: Infrared Spectroscopic and Theoretical Investigations of the OUF2 and OThF2 Molecules with Triple Oxo Bond Character.

The title molecules are prepared by reaction of OF₂ molecules with laser ablated uranium and thorium atoms in solid argon and neon.     

ChemInform Abstract: Matrix Infrared Spectroscopic and Theoretical of the Difluoroamino Metal Fluoride Molecules: F2NMF (M: Cu,Ag, Au).

The title molecules are obtained from spontaneous reactions of coinage metals and NF₃ in solid Ar and Ne matrices.     

Quantum Monte Carlo for the x-ray absorption spectrum of pyrrole at the nitrogen K-edge

Fixed-node diffusion Monte Carlo (FNDMC) is used to simulate the x-ray absorption spectrum of a gas-phase pyrrole molecule at the nitrogen K-edge. Trial wave functions for core-excited states are constructed from ground-state Kohn-Sham determinants substituted with singly occupied natural orbitals from configuration interaction with single excitations calculations of the five lowest valence-excited triplet states. The FNDMC ionization potential (IP) is found to lie within 0.3 eV of the experimental value of 406.1 ± 0.1 eV. The transition energies to anti-bonding virtual orbitals match the experimental spectrum after alignment of IP values and agree with the existing assignments.     

Reactive quenching of OH A (2)Σ(+) by O(2) and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels

The outcomes following collisional quenching of electronically excited OH A (2)Σ(+) by O(2) and CO are examined in a combined experimental and theoretical study. The atomic products from reactive quenching are probed using two-photon laser-induced fluorescence to obtain H-atom Doppler profiles, O ((3)P(J)) atom fine structure distributions, and the relative yields of these products with H(2), O(2), and CO collision partners. The corresponding H-atom translational energy distributions are extracted for the H + O(3) and H + CO(2) product channels, in the latter case revealing that most of the available energy is funneled into internal excitation of CO(2). The experimental product branching ratios show that the O-atom producing pathways are the dominant outcomes of quenching: the OH A (2)Σ(+) + O(2) → O + HO(2) channel accounts for 48(3)% of products and the OH A (2)Σ(+) + CO → O + HCO channel yields 76(5)% of products. In addition, quenching of OH A (2)Σ(+) by O(2) generates H + O(3) products [12(3)%] and returns OH to its ground X (2)Π electronic state [40(1)%; L. P. Dempsey, T. D. Sechler, C. Murray, and M. I. Lester, J. Phys. Chem. A 113, 6851 (2009)]. Quenching of OH A (2)Σ(+) by CO also yields H + CO(2) reaction products [26(5)%]; however, OH X (2)Π (v(") = 0,1) products from nonreactive quenching are not observed. Theoretical studies characterize the properties of energy minimized conical intersections in four regions of strong nonadiabatic coupling accessible from the OH A (2)Σ(+) + CO asymptote. Three of these regions have the O-side of OH pointing toward CO, which lead to atomic H and vibrationally excited CO(2) products and∕or nonreactive quenching. In the fourth region, energy minimized points are located on a seam of conical intersection from the OH A (2)Σ(+) + CO asymptote to an energy minimized crossing with an extended OH bond length and the H-side of OH pointing toward CO in a bent configuration. This region, exoergic with respect to the reaction asymptote, is likely to be the origin of the dominant O + HCO product channel.     

OMS, OM(η2-SO), and OM(η2-SO)(η2-SO2) molecules (M = Ti, Zr, Hf): infrared spectra and density functional calculations

Infrared spectra of the matrix isolated OMS, OM(η(2)-SO), and OM(η(2)-SO)(η(2)-SO(2)) (M = Ti, Zr, Hf) molecules were observed following laser-ablated metal atom reactions with SO(2) during condensation in solid argon and neon. The assignments for the major vibrational modes were confirmed by appropriate S(18)O(2) and (34)SO(2) isotopic shifts, and density functional vibrational frequency calculations (B3LYP and BPW91). Bonding in the initial OM(η(2)-SO) reaction products and in the OM(η(2)-SO)(η(2)-SO(2)) adduct molecules with unusual chiral structures is discussed.     

Experimental characterization of the CN X 2Σ+ + Ar and H2 potentials via infrared-ultraviolet double resonance spectroscopy

The hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ar complex with two quanta of CN stretch (v(CN) = 2), along with its ground state (v(CN) = 0), have been characterized by IR-UV double resonance and UV spectroscopy. Analysis of rotationally structured bands enable n(K) assignments and reveal perturbations due to Coriolis coupling between two closely spaced hindered rotor states, n(K) = 1(1) and 1(0). A deperturbation analysis is carried out to derive accurate rotational constants and their associated CN center-of-mass to Ar bond lengths as well as the magnitude of the coupling. The energetic ordering and spacings of the CN-Ar hindered rotor states provide a direct experimental probe of the angular dependence of the CN X (2)Σ(+) + Ar potential and permit radially averaged anisotropy parameters (V(10) = 5.2 cm(-1) and V(20) = 3.2 cm(-1)) to be determined. This analysis indicates a relatively flat potential about a linear N≡C-Ar configuration with a barrier to CN internal rotation of only ~12 cm(-1). The angular potentials determined from experiment and ab initio theory are in good accord, although theory predicts a higher barrier to CN internal rotation. A similar approach yields the infrared spectrum of H(2)-CN in the CN overtone region, which exhibits a rotationally resolved Σ ← Σ parallel band that is consistent with theoretical predictions for ortho-H(2)-CN.