Critical analysis of the spin-rotation constants of CF 2 and CCl 2 : A theoretical investigation Cristina Puzzarini a, * , Sonia Coriani b , Antonio Rizzo c , Ju ¨ rgen Gauss d a Dipartimento di Chimica ÔG. CiamicianÕ, Universita ` degli Studi di Bologna, Via Selmi 2, I-40126 Bologna, Italy b Dipartimento di Scienze Chimiche, Universita ` degli Studi di Trieste, Via Licio Giorgieri 1, I-34127 Trieste, Italy c Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche, Via G. Moruzzi 1, I-56124 Pisa, Italy d Institut fu ¨ r Physikalische Chemie, Universita ¨ t Mainz, D-55099 Mainz, Germany Received 5 April 2005; in final form 19 April 2005 Available online 23 May 2005 Abstract Quantum chemical ab initio calculations for the spin-rotation constants of difluorocarbene (CF 2 ) and dichlorocarbene (CCl 2 ) were carried out using coupled-cluster techniques with sequences of correlation-consistent basis sets. Theoretical best estimates were obtained using extrapolation to the complete basis-set limit and taking into account corrections for core correlation, additional diffuse functions and zero-point vibrational effects. It is demonstrated that such accurate theoretical estimates can be used either to support or to challenge the analysis of the experimental spectra and the reliability of the resulting data. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Quantum chemical ab initio calculations are nowa- days able to provide highly accurate results for molecu- lar energies and properties [1]. The predictive power of such computations is important in various fields (see, for examples [2–12]). Theoretical predictions play a par- ticularly fundamental role in the area of high resolution spectroscopy. For instance, it is well established that quantum chemical calculations can be used to reliably predict rotational spectra and that rotational and cen- trifugal distortion constants can be accurately evaluated (for recent examples, see [13,14]). Moreover, it is also possible to predict the fine- and/or hyperfine-structure of rotational spectra due to electric and/or magnetic interactions and to compute the therefore required nu- clear quadrupole coupling and/or spin-rotation con- stants rather accurately [14–16]. It should be pointed out that the quantum chemical evaluation of hyperfine parameters has a twofold impor- tance: first, such calculations allow to establish indepen- dent reference values for comparison with existing experiments and, second, they can provide reliable esti- mates for those parameters that are experimentally not available. As an example, in [16] ab initio computations strongly supported the experimental determination of the hyperfine parameters of several isotopomers of HCN: the theoretical values of the spin-rotation con- stants provided accurate predictions for the fit as well as reliable values for those constants not experimentally determinable, such as the constant for 15 N. In fact, it was shown that spin-rotation constants can be com- puted with an accuracy of about 0.2–0.3 kHz [16]. The present investigation is intended to demonstrate another exemplificative case, namely how quantum chemical ab initio calculations can be used to confirm as well as to challenge experimental data. In the following we will re- port state-of-the-art quantum chemical calculations for 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.099 * Corresponding author. Fax: +39 051 2099456. E-mail addresses: cristina.puzzarini@unibo.it (C. Puzzarini), coriani@units.it (S. Coriani), rizzo@ipcf.cnr.it (A. Rizzo), gauss@ uni-mainz.de (J. Gauss). www.elsevier.com/locate/cplett Chemical Physics Letters 409 (2005) 118–123