Microwave, r 0 structural parameters, conformational stability and vibrational assignment of cyclopropylcyanosilane James R. Durig a,⇑ , Gamil A. Guirgis b , Dattatray K. Sawant a,1 , Nathan A. Seifert c , Bhushan S. Deodhar a,2 , Brooks H. Pate c , Savitha S. Panikar a , Peter Groner a , Jason S. Overby b , Sahand M. Askarian b a Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA b Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA c Department of Chemistry , University of Virginia, Charlottesville, VA 22904, USA article info Article history: Received 5 July 2014 In final form 1 October 2014 Available online 13 October 2014 Keywords: Cyclopropylcyanosilane r 0 structural parameters Ab initio calculations Vibrational assignments Structural parameters Conformational stability abstract The FT-microwave spectrum (6000–19,800 MHz) of cyclopropylcyanosilane, c-C 3 H 5 SiH 2 CN has been recorded and 773 transitions for the 28 Si, 29 Si, 30 Si, 13 C and 15 N isotopomers have been assigned for cis and gauche conformers. Infrared spectra (3200–220 cm 1 ) of gas and Raman spectra (3200–40 cm 1 ) of the liquid have been recorded as well as the variable temperature (60 to 100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon. The enthalpy difference between the conformers in xenon solutions has been determined to be 123 ± 13 cm 1 (1.47 ± 0.16 kJ mol 1 ) with the cis conformer as the more stable form. Approximately 48 ± 2% of the cis form is present at ambient temperature. By uti- lizing the microwave rotational constants of six isotopomers for cis and seven isotopomers for gauche combined with the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r 0 parameters have been obtained for both conformers. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The determination of structural parameters and conformational stabilities are usually challenging for molecules like X–SiH 2 –Y (X = alkanes, cycloalkanes; Y = F, Cl, Br, CN and CH 3 ). Over the years, studies done on these molecules were very limited. We began our investigation with ethyl chlorosilane [1], ethyl fluorosi- lane [2] and ethyl bromosilane [3]. For ethyl fluorosilane and ethyl chlorosilane both gauche and trans conformers were observed in fluid phases and only the gauche conformer was observed in the solid phase. However, for ethyl bromosilane, the anti and gauche conformers were observed in the vapor and liquid phases and only the gauche form was observed in the solid phase [1–3]. By utilizing the variable temperature studies of the infrared spectra of samples dissolved in xenon solutions, the enthalpy difference was deter- mined to be 54 ± 16 cm 1 (0.65 ± 0.19 kJ/mol) for ethyl fluorosi- lane; 204 ± 23 cm 1 (2.44 ± 0.27 kJ/mol) for ethyl chlorosilane and 140 ± 25 cm 1 (1.67 ± 0.3 kJ/mol) for ethyl bromosilane with the gauche conformer being the most stable form in all three com- pounds [1–3]. We further investigated cyclopropylfluorosilane [4], cyclopro- pylchlorosilane [5] and cyclopropylbromosilane [6]. The enthalpy difference was determined to be 109 ± 9 cm 1 (1.30 ± 0.1 kJ/mol) for cyclopropylfluorosilane with the gauche form the most stable conformer, whereas for cyclopropylchlorosilane and cyclopropyl- bromosilane, the cis conformer was the more stable form with the enthalpy difference of 98 ± 10 cm 1 (1.2 ± 0.12 kJ/mol) and 126 ± 25 cm 1 (1.51 ± 0.30 kJ/mol), respectively, [4–6]. It is inter- esting to note that in cyclopropylchlorosilane and cyclopropylbro- mosilane both the gauche and cis conformers were present in the liquid phase and only the gauche conformer was present in the solid phase [5,6]. At ambient temperature, 23 ± 2% of cyclopropyl- fluorosilane was in the cis form, whereas for the cyclopropylbro- mosilane, the ratio of gauche to cis conformers was almost equal [4,5]. For cyclopropylsilane derivatives, conformational stability changes with respect to substitution of halogen in the silane moi- ety. The study of cyclopropylmethylsilane [7] showed that the gauche form was the most stable conformer with an enthalpy dif- ference of 98 ± 13 cm 1 (1.17 ± 0.16 kJ/mol), and at ambient tem- perature, the amount of cis conformer was 23 ± 6%. However, these data are not enough for establishing the relationship between conformational stability and cyclopropylsilane deriva- tives. Study of more similar molecules will be thus useful for http://dx.doi.org/10.1016/j.chemphys.2014.10.004 0301-0104/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +1 816 235 6038; fax: +1 816 235 2290. E-mail address: durigj@umkc.edu (J.R. Durig). 1 Taken in part from the thesis of Dattatray K. Sawant, which will be submitted in partial fulfillment of the Ph.D. degrees. 2 Taken in part from the thesis of Bhushan S. Deodhar, which will be submitted in partial fulfillment of the Ph.D. degrees. Chemical Physics 445 (2014) 68–81 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys