Spectroscopic studies of cryogenic fluids: Benzene in propane R. Nowak and E. R. Bernstein Citation: The Journal of Chemical Physics 86, 3197 (1987); doi: 10.1063/1.452030 View online: http://dx.doi.org/10.1063/1.452030 View Table of Contents: http://aip.scitation.org/toc/jcp/86/6 Published by the American Institute of Physics 8 Spectroscopic studies of cryogenic fluids: Benzene in propane ) b R. Nowak ) and E. R. Bernstein Department of Chemistry, Condensed Matter Sciences Laboratory, Colorado State University, Fort Collins, Colorado 80523 (Received 19 November 1986; accepted 8 December 1986) 1 Energy shifts and bandwidths for the 1B 2u ++ A Ig optical absorption and emission transitions of benzene dissolved in propane are presented as a function of pressure, temperature, and density. Both absorption and emission spectra exhibit shifts to lower energy as a function of density, whereas no shifts are observed if density is kept constant and temperature and pressure are varied simultaneously. Density is thus the fundamental microscopic parameter for energy shifts of optical transitions. The emission half-width is a linear function of both temperature and pressure but the absorption half-width is dependent only upon pressure. These results are interpreted qualitatively in terms of changes occurring in the intermolecular potentials of the ground and excited states. Both changes in shape of and separation between the ground and excited state potentials are considered as a function of density. Classical dielectric (Onsager­ Bottcher), microscopic dielectric (Wertheim) and microscopic quantum statistical mechanical (Schweizer-Chandler) theories of solvent effects on solute electronic spectra are compared with the experimental results. Calculations suggest limited applicability of dielectric theories but good agreement between experiment and microscopic theory. The results demonstrate the usefulness of cryogenic solutions for high pressure, low temperature spectroscopic studies of liquids. I. INTRODUCTION function of the solvent isothermal microscopic density. The Optical spectroscopic studies ofliquid solutions are con­ theory also predicts that the solute spectral shifts will de­ fronted with two problems: large spectral linewidths and pend on the detailed structure of the solvent and on solvent congestion due to hot bands. These problems have to some fluctuations. The validity of this approach has been dis­ extent been overcome by employing low temperature cryo­ cussed by its authors 12 based on a limited data set obtained 13 15 at 300 K. - genic liquids (Le., N 2 , CO, O2 , NF3 , CH4 , C2 H6 , C3 Hs, etc.) as solvents; these liquids have proven to be good sol­ The relative solute transition energy shifts calculated as vents yielding well resolved, relatively sharp (for the liquid a function of density are significantly different for dielectric phase) emission and absorption spectra of simple aromatic and microscopic (quantum statistical mechanical) theor­ 12 molecules. 1-7 Optical spectroscopic studies of aromatic sol­ ies. Additional detailed experiments and calculations are ute molecules in simple cryogenic liquids have been used to needed to elucidate quantitatively the deviations from di­ elucidate microscopic properties of liquid solutions, includ­ electric theories and the applicability of the microscopic ing an estimate of the solute-solvent intermolecular poten­ treatment. Traditionally, experiments have focused on vari­ 3 13-16; tials. ,5 These studies have focused on the comparison of able solvent and isothermal density studies more defini­ different solute-solvent systems in terms of gas to liquid tive tests of the theories would involve variable density (tem­ transition energy shifts, bandwidths, fluorescence and phos­ perature and pressure dependent) studies. Employing both phorescence lifetimes, solubilities, aggregate formation, and temperature and pressure changes for density variations, one their temperature dependences. should be able, in principle, to separate potential and kinetic Most of the existing theories which treat solvent effects energy increments to the total spectral behavior. on solute electronic spectra have been based on second order Cryogenic liquid systems are ideal for the above studies quantum mechanical perturbation calculations of the sol­ because they provide relatively sharp, well resolved solute ute-solvent dipolar intermolecular coupling. S-I I The classi­ spectroscopic features which can be obtained over a wide range of pressures and low temperatures. Thus, relatively cal nonpolar solvent shift theories lO,l1 correlate spectral transition energy shift with the macroscopic solvent optical large density variations can be achieved for these systems dielectric constant (refractive index) but do not relate spec­ without resorting to excessively high pressures. tral behavior directly with any microscopic parameters. In this paper we report the results of optical spectro­ scopic studies and theoretical calculations for benzene dis­ Schweizer and Chandler (S-C) 12 have recently devel­ oped a quantum statistical mechanical perturbation theory solved in liquid propane. Propane has been chosen for the which relates the solute transition energy shift to the solu­ initial studies for a number of reasons. First, spectroscopic tion density and the polarizabilities of the solute and solvent behavior of different organic molecules (e.g., benzene, naph­ molecules. The important conclusion of this theory is that thalene, toluene, and pyrazine) in propane at low tempera­ the relative solute transition energy shifts are an essential ture and atmospheric pressure is well known from our pre­ I 7 vious work. - Second, propane has well established solvation properties with respect to benzene; no aggregates oj Supported in part by a grant from NSF. are found for benzene/propane solutions. Third, propane is b) On leave from the Institute of Organic and Physical Chemistry, Technical University ofWroclaw, 50-370 WrocJaw, Poland. a good liquid pressurizing medium: it has a high critical tem- J. Chern. Phys. 86 (6), 15 March 1987 0021-9606/87/063197-10$02.10 @) 1987 American Institute of Physics 3197 3198 R. Nowak and E. R. Bernstein: Spectroscopy of benzene in propane perature (369.85 K) and low critical pressure (4.247 MPa) ates in the pressure range from 10-6 Torr to 4 kbar at tem­ and well known thermodynamic data. Both pressure and peratures from 70 to 300 K without the need for frequent temperature dependences of the density and dielectric con­ packing replacement. The pressurizing system consists of 17 19 stant - are available: these data are important for setting hand operated generators and valves (High Pressure Equip­ up experimental conditions and for performing theoretical ment Corp.) and is connected to a high vacuum pumping calculations. system. Pressure is measured by resistance pressure trans­ Moreover, benzene has previously proved to be a good ducers (Sensotec) to an accuracy of ± 20 bar. solute molecule for the probing of liquid state interactions All chemicals are obtained in the highest available com­ and structure. 1.3.4 Benzene has high symmetry, well known mercial purity and used without additional purification. Gas spectra and relaxation dynamics, and good solubility. The samples for deposit in the precooled cell are prepared by intense vibronic series 6~ I~ and 6~ I~, n = 0,1,2,3, ... of the mixing solute and solvent at room temperature. Benzene benzene IB2u~lAlg transition can be observed in both ab­ concentration (typically 50-200 ppm) is controlled by tem­ sorption and emission and can be readily followed as a func­ perature variation of its vapor pressure in a known volume. tion of temperature and pressure in liquid propane. The entire system is filled with this mixture or pure propane The well resolved absorption and emission spectra of which acts as the pressurizing fluid. The number of benzene benzene in liquid propane are determined as a function ofthe molecules in the optical path changes by about 20% due to theoretically important parameters temperature, pressure, the density change over the course of the experiment. Addi­ and density. Comparing both absorption and emission be­ tionally, the solubility of benzene in propane is both tem­ havior, we are able to estimate changes in the intermolecular perature and pressure dependent. These effects are ,in part potential surfaces in both the ground and excited electronic responsible for the apparent change in transition intensity states. To the best of our knowledge this is the first report of with pressure. We assume that these concentration changes electronic emission spectra in a solute-solvent system as a are not important for the interpretation of transition energy function of pressure. The comparison of the experimental shifts and transition half-widths; however, they do obscure data with dielectric and microscopic theories demonstrates comparison of the absolute and relative intensities of the fea­ the limited applicability of dielectric theories and the accura­ tures at different experimental conditions. cy of the microscopic quantum statistical mechanical treat­ ment. III. THEORY OF SOLVENT EFFECTS ON SOLUTE ELECTRONIC SPECTRA II. EXPERIMENTAL PROCEDURES The following presentation is limited only to those theo­ The optical absorption and dispersed emission spectra retical results which are pertinent to the interpretation ofour of the benzene IB2u~IAlg transition are studied as a func­ experimental energy shifts. Specifically,