The vibrational spectrum of camphor from ab initio and density functional theory and parity violation in the C–C*–CO bending mode Peter Schwerdtfeger a, * , Andreas K€ uhn a , Radovan Bast a , Jon K. Laerdahl b , Francesco Faglioni c , Paolo Lazzeretti c a Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Chemistry, University of Oslo, P.O.Box 1033, Blindern, N-0315 Oslo, Norway c Dipartimento di Chimica, Universit a degli Studi, via G. Campi 183, 41100 Modena, Italy Received 6 May 2003; in final form 12 November 2003 Published online: Abstract The C–C*–CO bending mode in camphor (C* denotes a chiral carbon) as originally investigated for parity violation effects by Arimondo et al. [Opt. Commun. 23 (1977) 369] lies in the CO 2 laser frequency range with the most intense transition at 1045 cm 1 . The vibrational spectrum of camphor is, therefore, reinvestigated by ab initio and density functional calculations and compared with experimental results. This provides the basis for a local mode analysis which reveals that the C–C*–CO bending mode has only minute parity violation contributions (<10 5 Hz) to vibrational transitions far below the current detection limit of high resolution spectroscopy. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Attempts to measure parity violation (PV) in the vi- brational spectra of chiral compounds have a long hi- story [1–6]. Shortly after Rein [7] stated in 1975 that the direct observation of energy differences between optical isomers is probably beyond the scope of present exper- imental facilities Arimondo et al. [2] made the first at- tempt to find differences in transition frequencies of d - and l-camphor using the 9.22 lm R(28) CO 2 laser line, which conveniently lies in the C–C*–CO bending mode range of camphor (C* denotes a chiral carbon in the camphor molecule). They concluded that PV effects in camphor must be smaller than 300 kHz. Recent ex- periments by Chardonnet and co-workers [3] focused on the C–F stretching mode in chiral carbon compounds. Using saturation spectroscopy in the 9.3 lm spectral range with a tuneable CO 2 laser they reported Dm PV ¼ 4:2  0:6  1:6 Hz (including statistical and systematic uncertainties) for this mode in CHFClBr, which is due to collision effects under the experimental conditions [8]. All attempts to measure the influence of PV effects on frequency shifts between enantiomers have failed so far [9]. Current estimates set such effects in the range of a few milliHertz [10–12], and are, therefore, several orders of magnitude below current high resolution spectros- copy (about 1 Hz). Since the experiments of Arimondo et al. [2] are of historical importance, we decided to reinvestigate the vibrational spectrum of camphor and estimate PV effects in the 1045 cm 1 band assigned originally to the C–C*– CO bending mode from first-order perturbation theory, E PV;n ¼ W n E PV j jW n h i; ð1Þ where W n denotes the vibrational wavefunction for a specific vibrational level n, and E PV is the parity viola- tion energy surface, i.e., the PV correction term to the complete potential energy surface. As it is well known Chemical Physics Letters 383 (2004) 496–501 www.elsevier.com/locate/cplett * Corresponding author. Fax: +(649)-3737422. E-mail addresses: p.schwerdtfeger@auckland.ac.nz, schwerd@ ccu1.auckland.ac.nz (P. Schwerdtfeger). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.035