The structure of 1-formyl-3-phenyl-D 2 -pyrazoline in the gas phase (DFT calculations), in solution (NMR) and in the solid state (X-ray crystallography) I. Alkorta a , J. Elguero a , A. Fruchier b , N. Jagerovic a, * , G.P.A. Yap c, * a Instituto de Quı ´mica Me ´dica, CSIC, Juan de la Cierva, 3, E-28006 Madrid, Spain b UMR 5076, E.N.S.C.M., 8 rue de l’Ecole Normale, F-34296 Montpellier Ce ´dex 5, France c Department of Chemistry and Biochemistry, University of Delaware, Newark DE 19716, USA Received 22 September 2003; revised 28 October 2003; accepted 28 October 2003 Abstract The molecular structure of 1-formyl-3-phenyl-D 2 -pyrazoline was determined by X-ray crystallography (triclinic, P-1). The geometry thus obtained was compared with that obtained by DFT calculations. The GIAO method was used to calculate absolute shieldings, which agree conveniently with those measured by 13 C and 15 N NMR. The title compound appears to be an essentially planar molecule. q 2003 Elsevier B.V. All rights reserved. Keywords: Pyrazoline; Crystal structure; 13 C NMR; 15 N NMR; DFT; GIAO calculations 1. Introduction In 1965 we reported the synthesis of 1-formyl-3-phenyl- D 2 -pyrazoline 1 by reaction between formic acid and 3-phenyl-D 2 -pyrazoline in benzene at reflux eliminating water with a Dean-Stark trap [1]. A pale yellow solid was isolated, mp 102 8C. Afterwards a 1 H NMR study using Aromatic Solvent Induced Shifts (ASIS) allows determining that the major isomer in solution was the E one (1E) [2]. We have already reported the 1 H NMR data of compound 1E [3,4], compound that has only been described once by other authors [5]. A series of 1-formyl-D 2 -pyrazoline have been studied by Zelenin and coworkers [6]. All these compounds exist also in the E conformation (from 1 H NMR data in CDCl 3 ). D 2 -Pyrazolines, like cyclopentene, are not planar but present an envelope conformation with the C5 carbon out of plane. In order to determine the folding of the envelope, we used two modified Karplus approaches [3]. The first one uses the difference between the vicinal coupling constants ðDJ f Þ and the second one their ratio ðRJ f Þ; both taking into account the electronegativity of N1 and the parallelism p – s between the lone pair of N1 and the C–H bond [3]. We showed that the AA 0 BB 0 of 1 can be completely analysed only at 60 MHz because some important tran- sitions are lost at 100 MHz or higher fields: 3 J HH ðcisÞ¼ 12:35 Hz and 3 J HH ðtransÞ¼ 7:65 Hz. [3]. Then, considering that the equilibrium constant between the two folded structures has a value of 1 because they are enantiomers and assuming a perfect 1,2-substituted ethane fragment (R–CH 2 –CH 2 –R 0 ), we calculated using the DJ f approach a cis dihedral H–C–C–H angle of 188 and using the RJ f approach an angle of 338. We decided to determine the E=Z conformation and the folding of compound 1 in the solid state, to complete its 0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2003.10.041 Journal of Molecular Structure 689 (2004) 251–254 www.elsevier.com/locate/molstruc * Corresponding authors. E-mail addresses: nadine@iqm.csic.es (N. Jagerovic), gpyap@udel.edu (G.P.A. Yap).