Matrix-Isolation FTIR Spectroscopy of Benzil:  Probing the Flexibility of the C−C Torsional Coordinate

Abstract

The infrared spectrum and conformational flexibility of benzil, (C 6 H 5 CO) 2 , are studied by matrix-isolation FTIR spectroscopy, supported by DFT calculations. It is shown that the low-frequency (ca. 25 cm -1 ), largeamplitude torsion around the C-C central bond strongly affects the structural and spectroscopic properties exhibited by the compound. The equilibrium conformational distribution of molecules with different OdC-CdO dihedral angles, existing at room temperature in the gas phase, and trapped in a low-temperature (T ) 9 K) inert matrix can be changed either by in situ irradiation with UV light (λ > 235 nm) or by annealing the matrix to higher temperatures (T ≈ 34 K). In the first case, the increase of the average OdC-CdO angle results from conformational relaxation in the excited electronic states (S 1 and T 1 ), whose lowest-energy conformations correspond, for both S 1 and T 1 states, to a nearly planar configuration with the OdC-CdO dihedral angle equal to 180°. In the second case, the decrease of the average value of the OdC-CdO dihedral angle is a consequence of the change in the S o C-C torsional potential, resulting from interactions with the matrix media, which favors the stability of the more polar structures with smaller OdC-CdO dihedral angles.

Molecular Structure, Vibrational Spectra and Photochemistry of 2-Methyl-2H-Tetrazol-5-Amine in Solid Argon

“…Stabilization of more planar structures in matrixes (when compared with gas phase) is a relatively frequent phenomenon; it was observed previously, for example, for cyanoacetic acid and methyl cyanoacetate 35,36 and benzil. 37 …”

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

“…This range of values is typical for a non-conjugated C C single bond and follows the trend previously observed for other ␣-dicarbonyls, like diacetyl, benzil, 1-phenyl-1,2-propanedione and ␣-furil (155.7, 154.4, 155.0 and ca. 154.7 pm, respectively) [1][2][3][4]. As discussed in detail elsewhere [4], the long intercarbonyl bond length in ␣-dicarbonyls results essentially from the balance between the relatively weak -electron delocalization within the O C C O fragment and the more important -electron system repulsion due to the interaction between the positively charged carbonyl carbon atoms.…”

“…In addition, some insights regarding the prevalent intramolecular interactions in the different conformers can also be obtained by comparing the geometries of ␣-pyridil and some analogous compounds, like diacetyl [1], benzil [2] and ␣-furil with those found in other ␣-dicarbonyl molecules previously studied in our laboratory, such as diacetyl 1 shows that the value of the intercarbonyl dihedral angle depends strongly of the structure of the substituent connected to the carbonyl groups. In acetyl, the substituents are small and the prevalent interaction is the repulsion between the lone electron pairs of the oxygen atoms, leading to a most stable trans configuration around the central bond [1].…”

“…In general, simple ␣-dicarbonyl compounds show interesting structural, spectroscopic and photophysical properties, including photorotamerism, due to conformational changes associated with the internal rotation around the flexible O C C O intercarbonyl torsional coordinate in the ground or excited electronic states [1][2][3][4][5][6][7][8]. Because of that, these compounds have received particular attention concerning their potential use for the development of molecular devices [9][10][11][12][13][14].…”

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2 ] has been isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. Calculations predicted the existence of three different conformers exhibiting skewed conformations around the intercarbonyl bond and the two C 5 H 4 N C( O) fragments nearly planar.

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