Vibrational frequency assignment and molecular structure of p-benzoquinone

Abstract

The infra-red absorptions of p-benzoquinone (O=C6H4=O) have been studied in the vapour, solution and solid phases. Measurements were made in the range 5000-200 cm-1: In conjunction with published Raman spectra and our own infra-red spectra for a number of simple derivatives, including deutero-bcnzoquinone (O=C,sD4=O) and the di-imine (HN=Cs-=NH), a complete assignment of the fundamental modes is presented. A self-consistent m.0. calculation of the bonding conditions is brie5y reported.As expected, the frequency pattern shows a number of well-defined differences from benzenoid structures, and it is suggested that these can assess the occurrence of mixed benzenoidquinonoid character in simple aromatic structures, e.g., in pC6H4F2. A feature of the benzoquinone spectrum in the nearly saturated solution or crystalline state is the rich sequence of combination tones which do not appear in the vapour : in the crystal it is the result of coupling (possibly electron delocalization?) between different molecules. para-Benzoquhone is the parent structure for a large range of quinonoid-type of molecules, including many impo'rtant dyes, Whilst the benzenoid vibrational frequencies have been extensively studied and much used in infra-red and Raman investigations, the corresponding quinonoid features are far less well defined. In view of the many and ready uses of such characteristic frequencies and especially their possible use in assessing the extent of resonance in a benzenoidquinonoid structure, the assignment of the vibrational frequencies of para-benzoquinone is of special interest.The characterization of the infra-red frequencies in para-disubstituted benzene derivatives has been made by several authors l a v * ~ 2-4. However, a complete study of para-benzoquinone has not been made. Several investigations concerning the carbonyl stretching frequency can be found 5 and Anno and Sado 5 and Claverie 6have made partial assignments of the other frequencies. The Raman spectrum of the paraquinone was first obtained by Kohlrausch, Pongratz and Seka.7 This was improved by Stammreich and Forneris.8 EXPERIMENTALThe spectrometers used in this study were the Grubb Parsons' G.S.2 grating spectrometer for the region 2-15 p, S.3 prism instrument with KBr optics for the region 15-22 p and D.M.2 grating instrument for the 20-5Op range: no absorptions were found below 400 cm-1 (25 p).Para-benzoquinone was prepared,g steam-distilled, and recrystallized from benzene ; m.p. 11 5.0-1 15.2OC. Deutero-benzoquinone was prepared by refluxing hydroquinone with D20 in the presence of H2SO4. This mixture was cooled to 0°C and oxidized by the slow addition of a concentrated solution of sodium dichromate in D20. The deuteroquinone was extracted with carbon tetrachloride and cyclohexane; m.p. (in sealed tube) 113.5-1140°C. The extent of deuteration was about 83 % (e.g., 0.32[C@402]+0-68[Cd33H02l). MOLECULAR STRUCTUREThe most recent structural determination is a three-dimensional andysis of the crystal by Trotter9 This work contains reference to the othe...

Vibronic spectra of the p-benzoquinone radical anion and cation: a matrix isolation and computational study

“…26,30,31 The assignment of this spectrum, in particular that of the pair of intense bands at 1650-1670 cm…”

“…This spectrum is in excellent accord with previous measurements in the gas and in condensed phases. 26,30,31 The assignment of this spectrum, in particular that of the pair of intense bands at 1650-1670 cm À1 , which appear prominently in the matrix spectrum, had been controversial, because calculations predict only one fundamental in this region. However, it was established that the two bands which are peaking at 1659 and 1671 cm À1 in our matrix spectra are due to two Fermi-coupled b 1u vibrational states.…”

Thermodynamic Properties of 1,4-Benzoquinones in Gaseous and Condensed Phases: Experimental and Theoretical Studies

“…The recommended frequencies in ref 10 are composed of the experimental spectra of gaseous and liquid p-Qu as well as its spectra in various solvents, 25,26 wherein, the change of solvent leads to a significant change in the frequencies of p-Qu. 27,28 At the same time, the calculations by the authors themselves 10 according to the use of valenceforce field 29−31 showed a significant discrepancy between the theoretical frequencies and ω exp measured in the liquid phase, specifically for the high amplitude motion. In work 32, the farinfrared spectrum of gaseous p-Qu was measured.…”

“…As noted before, they calculated the entropy S ° , the heat capacity C p , the reduced enthalpy ( H T 0 – H 0 0 )/ T and the reduced free energy −( G T 0 – H 0 0 )/ T within the scope of the RRHO model in the range 273.15–1500 K (Supporting Information) using this set of frequencies. The recommended frequencies in ref are composed of the experimental spectra of gaseous and liquid p -Qu as well as its spectra in various solvents, , wherein, the change of solvent leads to a significant change in the frequencies of p -Qu. , At the same time, the calculations by the authors themselves according to the use of valence-force field − showed a significant discrepancy between the theoretical frequencies and ω exp measured in the liquid phase, specifically for the high amplitude motion. In work , the far-infrared spectrum of gaseous p -Qu was measured.…”

Radiative Cooling of Surface-Modified Gold Nanorods upon Pulsed Infrared Photoexcitation

“…The assignments of the neat capping materials are discussed in section 3 of the Supporting Information. The spectrum of AuNR@CTAB (Figure a) manifested most of the characteristics of CTAB, with some residual absorbances, which might be attributed to the most intense bands of the CC stretch mode (red fragment at ∼1515 cm –1 ) of the reactant hydroquinone and the CO stretch mode (green fragment at ∼1680 cm –1 ) of the oxidation product p -benzoquinone in the synthesis. Some weak bands were buried at 1300–1000 cm –1 .…”