Barbara M. Giuliano

Infrared spectra and ultraviolet-tunable laser induced photochemistry of matrix-isolated phenol and phenol-d5.

Monomers of phenol and its ring-perdeuterated isotopologue phenol-d(5) were isolated in argon matrices at 15 K. The infrared (IR) spectra of these species were recorded and analyzed. In situ photochemical transformations of phenol and phenol-d(5) were induced by tunable UV laser light. The photoproducts have been characterized by IR spectroscopy supported by theoretical calculations of the infrared spectra. The primary product photogenerated from phenol was shown to be the phenoxyl radical. The analysis of the progress of the observed phototransformations led to identification of 2,5-cyclohexadienone as one of the secondary photoproducts. Spectral indications of other secondary products, such as the Dewar isomer and the open-ring ketene, were also detected. Identification of the photoproducts provided a guide for the interpretation of the mechanisms of the observed photoreactions.

Infrared Spectra and Photochemistry of Matrix-Isolated Pyrrole-2-carbaldehyde

Monomeric pyrrole-2-carbaldehyde (P2C) was isolated in low-temperature argon and xenon matrices, and its UV-induced photochemistry was studied. The structures of the reagent as well as the reaction photoproducts were characterized by FTIR spectroscopy. Interpretation of the experimental results was assisted by theoretical calculations carried out at the MP2 and DFT (B3LYP) levels with the 6-311++G(d,p) basis set. The compound can assume two conformations, cis and trans, regarding the orientation of the N-C-C=O dihedral angle. The cis form is the conformational ground state, being more stable than the trans by ca. 15 kJ mol(-1). The relative stability of the two conformers was analyzed based on the comparison of their structures and using the natural bond orbital method. In agreement with the calculations, only the signature of the cis conformer was found in the experimental FTIR spectra of matrix-isolated P2C monomers. UV irradiation (lambda > 235 nm) readily converts the cis-P2C into the trans-P2C form, and a photostationary equilibrium is established where the [cis]/[trans] ratio is ca. 3.3:1 in both Ar and Xe. Upon prolonged irradiation, P2C slowly undergoes photolysis to [pyrrole + CO]. In matrices, pyrrole and CO form associates of different geometry, which could be characterized based on their vibrational signatures.

Features of the C-H...N weak hydrogen bond and internal dynamics in pyridine-CHF3.

The structural and energetic features of the C H···N interaction and the internal dynamics of the pyridine–trifluoromethane molecular complex in its normal, N and C species are here reported. They have been obtained from analysis of the pure rotational spectra of the complex generated in a supersonic expansion. Weak hydrogen bonds (WHB) are a major topic in hydrogen-bond research. The energies of these interactions lie within a few kJ mol 1 and approach those of van der Waals forces, but have the same directional properties of “classical” hydrogen bonds. This behavior was recently shown for the C H···O, C H···F C, C H···S, and C H···p linkages by investigating the rotational spectra of several hydrogenbonded molecular complexes generated in supersonic jets. With this technique, precise information on the energetics and on the structural and dynamical aspects of such interactions is obtained in an environment free from the intermolecular interactions that take place in condensed media. Investigation of the rotational spectrum of benzene–trifluoromethane has shown that this complex is a symmetric top, with the two moieties held together through a C H···p interaction, and thus provided information on this kind of WHB. When replacing benzene with pyridine, two sites of high electronic density become available in the ring, so the two adducts shown in Figure 1 could be plausibly formed for the pyridine–trifluoromethane (Py–CHF3) complex.

Molecular Recognition of Chiral Conformers: A Rotational Study of the Dimers of Glycidol

Two homochiral dimers of glycidol, deriving from two different conformers, have been characterized by rotational spectroscopy in a supersonic expansion.

Tautomerism in 4-Hydroxypyrimidine, S-Methyl-2-thiouracil, and 2-Thiouracil

The keto-enol tautomerism of 4-hydroxypyrimidine and of the related molecules S-methyl-2-thiouracil and 2-thiouracil has been investigated using synchrotron-based techniques. The populations of the constituent tautomers and thermodynamic parameters have been obtained by analysis of core-level photoemission spectra. The effect of substituents on the stability of tautomers has been revealed. Attaching additional OH (or SH) groups to the aromatic ring stabilizes the dioxo (or oxo-thione) forms. However, substitution of hydrogen in position 2 by an S-CH(3) group (that is, in going from 4-hydroxypyrimidine to S-methyl-2-thiouracil) does not significantly affect the tautomeric equilibrium.

Conformational Behavior and Tautomer Selective Photochemistry in Low Temperature Matrices: The Case of 5-(1H-Tetrazol-1-yl)-1,2,4-triazole

The conformational properties and the photolysis behavior of one of the simplest N-C bonded bicyclic azoles, 5-(1H-tetrazol-1-yl)-1,2,4-triazole (T), were studied in argon and xenon matrices by infrared spectroscopy. Analysis of the experimental results was supported by extensive theoretical calculations carried out at the B3LYP/6-311++G(2d,2p) level of approximation. Out of the eight T minima located on the potential energy surface, the three most stable species were detected in low temperature matrices, namely, 5-(1H-tetrazol-1-yl)-1H-1,2,4-triazole (T1) and two conformers of 5-(1H-tetrazol-1-yl)-2H-1,2,4-triazole (T2a and T2b). With increase of the substrate temperature either during deposition of the matrices or during annealing the T2b → T2a conversion took place, in agreement with the predicted low energy barrier for this transformation (5.38 kJ mol(-1)). Both broad band and narrow band laser UV irradiations of T isolated in Xe and Ar matrices induce unimolecular decomposition involving cleavage of the tetrazole ring of T1 and T2a (T2b) that leads to the production of 1H-1,2,4-triazol-5-yl carbodiimide (P1) and 1H-1,2,4-triazol-3-yl carbodiimide (P2), respectively. When the laser is used, in addition to the main P1 and P2 photoproducts, several minor products could be successfully identified in the matrices: N-cyanocarbodiimide HNCNCN (detected for the first time) associated with nitrilimine HNNCH and HCN. An interesting phenomenon of tautomer-selective photochemistry was observed for the matrix-isolated compound. It could be explained by the different LUMO-HOMO energy gaps estimated for T1, T2a, and T2b, connected with different threshold energies necessary to start the photolysis of T1 and T2a (T2b).

Adducts of NH3 with the Conformers of Glycidol: A Rotational Spectroscopy Study

Various chemical problems, difficult to unravel with other techniques, were solved recently by rotational spectroscopy experiments in supersonic jets. The investigations of molecular complexes with this technique gave indications on the nature of the forces which bind the constituent molecules, on the preferred sites of interaction, and on the widely unexplored interactions in the intermediate regime between bonding and nonbonding. Molecular recognition, molecular aggregation, rare-gas atoms forming covalent bonds, and many more phenomena are investigated with this techniques without having to rely heavily on ab initio calculations— although their support is of great help in guiding spectroscopic searches and interpreting the spectra. Many of the complexes investigated involve one or more molecules of water linked to organic or biomolecules. Water participates in several kinds of hydrogen bonds, acting either as a proton acceptor or a proton donor. The most common hydrogen bonds in these systems are of the type O Hw···O, O H···Ow, O Hw···N, N H···Ow, where the subscript indicates an atom belonging to the water molecule. These hydrogen bonds are moderately strong, in the range 15–25 kJmol . Less information is available on molecular complexes of ammonia with organic molecules. The only systems investigated in which ammonia could play the dual proton-donor/ proton-acceptor role are F3CH···NH3, [5] CH3OH···NH3, [6]

Rotational and Core Level Spectroscopies As Complementary Techniques in Tautomeric/Conformational Studies: The Case of 2-Mercaptopyridine

Millimeter wave free jet absorption and core level photoemission spectroscopies give complementary and precise information on the conformational/tautomeric equilibrium of 2-mercaptopyridine.

The rotational spectra, potential function, Born-Oppenheimer breakdown, and magnetic shielding of SnSe and SnTe

The pure rotational spectra of 27 isotopic species of SnSe and SnTe have been measured in the frequency range of 5-24 GHz using a Fabry-Perot-type resonator pulsed-jet Fourier-transform microwave spectrometer. Gaseous samples of both chalcogenides were prepared by laser ablation of suitable target rods and were stabilized in supersonic jets of Ar. Global multi-isotopolog analyses of all available high-resolution data produced spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as Born-Oppenheimer breakdown (BOB) coefficients delta01 for Sn, Se, and Te. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both SnSe and SnTe. Additionally, the magnetic hyperfine interaction produced by the dipolar nuclei 119Sn, 117Sn, 77Se, and 125Te was observed, yielding first determinations of the corresponding spin-rotation coupling constants.

Tautomers and UV-Induced Photoisomerization of a Strongly Intramolecularly H-Bonded Aromatic Azo-Dye: 1-(Cyclopropyl)diazo-2-naphthol

Aromatic azo compounds have a wide range of industrial applications as dyes in optical and color-changing materials and can also be exploited in the design of new photodynamic molecular systems. The azo derivative 1-(cyclopropyl)diazo-2-naphthol was isolated in low-temperature cryogenic matrices, and its molecular structure, tautomeric equilibrium, and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. Only azo-enol forms having the OH group involved in a strong intramolecular hydrogen bond, forming a six-membered ring with the azo group, were found experimentally. Irradiation with a narrowband source in the near-UV range generates different rotameric and tautomeric azo-enol and keto-hydrazone forms that can be interconverted at different irradiation wavelengths.