Infrared Absorption Studies of n-Heptane under High Pressure Masashi Yamaguchi, Scott V. Serafin, Thomas Hellman Morton, and Eric L. Chronister* Department of Chemistry, UniVersity of California, RiVerside, California 92521-0408 ReceiVed: September 25, 2002; In Final Form: December 9, 2002 Mid-infrared spectra of neat n-heptane at room temperature are presented over a pressure range from ambient to 70 kbar. The application of hydrostatic pressure induces frequency shifts, band splittings, and significant changes in the line shapes of internal vibrational modes both in liquid and in solid phases. The results are discussed in terms of the liquid-solid phase transition and changes of the population of molecular conformers. Evidence for a solid-solid phase transition near 30 kbar is also presented. Introduction The phase behavior and the conformational equilibria of simple saturated hydrocarbons have attracted considerable atten- tion for many years and have been investigated extensively. 1-6 Standard textbooks point out the effects of molecular symmetry, whereby the melting points of n-alkanes with even and odd numbers of carbons trace out two distinct curves. The odd homologues melt at lower temperatures than would be predicted by interpolating between even homologues, a result attributed to the fact that, when fully extended, chains with an even number of carbons possess inversion symmetry, while chains with an odd number of carbons do not. 7 As a consequence, the former pack better in a crystal than do the latter. By contrast, no such differences between even and odd n-alkanes are apparent in the liquid phase, because the boiling points increase in a smooth progression with increasing molecular weight. The properties of alkyl chains are fundamentally important for understanding a variety of phenomena in biology and polymer science, such as the phase behavior of highly ordered lipid-hydrocarbon chain assemblies 8,9 and the morphology and crystal growth of polymers. 10,11 Vibrational spectroscopy is a powerful tool for investigating conformation and dynamics of molecules in condensed phases and has been used ex- tensively. 12-14 Application of hydrostatic pressure to molecular systems that are held together by van der Waals interactions modifies the balance of the intra- and intermolecular forces as a result of changes in the distances between molecules. For linear alkanes, which have many intramolecular degrees of freedom, the application of pressure perturbs the distribution of molecular conformations in the liquid phase. 3,12,15,16 It has been proposed that the application of pressure increases the population of globular conformers with gauche bonds. Schoen et al. 15 reported high-pressure Raman experiments on liquid n-hexane, n-heptane, and n-octane, all of which exhibited a pressure dependence of the Raman intensity of longitudinal acoustic modes. From their data, they deduced the pressure dependence of the molecular conformational populations. Kato et al. 12 reported the pressure dependence of infrared spectra of n-pentane and discussed its conformational equilibrium in terms of the spectroscopic line shape changes for the methyl rocking mode in the liquid phase. In addition, the pressure dependence of the low-temperature dynamics of dye-doped solid n-alkane crystals has been studied by hole burning experiments, and it has been reported that they have unusually large pressure-induced broadening. 17 Investiga- tion under high pressure is desirable to further understand the phase behavior as well as the dynamics of these molecular systems. In this article, we present the results of an infrared study of n-heptane under high pressure. The n-heptane molecule has a sufficiently long chain to take on a variety of conformations, some of which are spectroscopically distinguishable. We have investigated the effect of pressure on the infrared spectrum of n-heptane in the liquid and solid phase up to 70 kbar by using a diamond anvil cell, and we interpret the results with the help of a density functional theory (DFT) survey of low-lying molecular conformations. The pressure-induced frequency shifts, band splittings, and changes in line shape are discussed in terms of the liquid-solid phase transition and the population of conformational isomers. Various nomenclature systems have been used to describe chain conformations. The two most commonly seen orientations are represented at the center and left in Chart 1, and three different designations for each are summarized. When the substituents X and Y are CH 2 or CH 3 groups, the nonbonded interactions between their hydrogens are repulsive, and the synclinal orientation is said to be gauche. This paper will follow a recently proposed recommendation that the ≈180° orientation be abbreviated as A and the ≈60° orientation be abbreviated as G. In some cases, the dihedral angle is closer to 90°. The proposed nomenclature for such an orientation is ortho (abbrvi- ated as O). 18 Note that the gauche and ortho geometries cannot be superimposed on their mirror images. Hence, the helical sense * Author to whom correspondence should be addressed. E-mail: eric.chronister@ucr.edu. Phone: 909-787-3288. Fax: 909-787-3420. CHART 1. Orientations about a Single Bond 2815 J. Phys. Chem. B 2003, 107, 2815-2821 10.1021/jp0221439 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/06/2003