Communication 15 N– 15 N spin–spin coupling constants through intermolecular hydrogen bonds in the solid state Rosa M. Claramunt a, * , Marta Pérez-Torralba a , Dolores Santa María a , Dionisia Sanz a , Bénédicte Elena b , Ibon Alkorta c , José Elguero c a Departamento de Química Orgánica y Bio-Orgánica, Facultad de Ciencias, UNED, Senda del Rey 9, E-28040 Madrid, Spain b Université de Lyon, Centre de RMN à très hauts champs, CNRS/ENS Lyon/UCBL, 5 rue de la Doua, 69100 Villeurbanne, France c Instituto de Química Médica (CSIC), Juan de la Cierva 3, E-28006 Madrid, Spain article info Article history: Received 18 May 2010 Revised 13 July 2010 Available online 30 July 2010 This paper is devoted to Professor José Manuel Concellón who untimely passed away. Keywords: Pyridine Pyridinium Hydrogen bond J couplings Solid-state NMR spectroscopy abstract A 2h J NN intermolecular spin–spin coupling constant (SSCC) of 10.2 ± 0.4 Hz has been measured for the powdered tetrachlorogallate salt of pyridinium solvated by pyridine (pyridine–H + pyridine cation 3). Density Functional Theory (DFT) calculations at the B3LYP/6-311++G(d, p) level reproduced this value and two others reported in the literature for 2h J intermolecular SSCCs, which were measured for com- plexes in solution. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Interest in the measurement of scalar NMR spin–spin coupling constants across hydrogen bonds is considerable due to their rele- vance for the structures of biomolecules. The experimental diffi- culty has limited most of these measurements, either in solution or in solids, to the 1/2 spin nuclei 1 H, 13 C, 15 N and 19 F, with few data involving 31 P; couplings with biologically interesting nuclei, such as 17 O, have been measured recently, for instance a 2h J NO has been measured in uracil [1]. To determine a coupling constant across a hydrogen bond (HB), several conditions must be fulfilled: (i) the HB strength must be moderate to high, which limits most studies to NH, OH and FH as HB donors and O(sp 2 ) and N(sp 2 ) as HB acceptors; and (ii) the geometry of the HB must be rigid. This latter requirement is not a problem if the HB is intramolecular, but if intermolecular this condition requires either biomolecules (nucleic acids, proteins) held together by multiple HBs or low-tem- perature studies. In the case of X–HY complexes, the three con- stants 1 J XH , 1h J HY and 2h J XY can be measured if X and Y are 1/2 spin nuclei, but if one of the nuclei has a I > 1/2, for example, X = O and Y = N, then in general only one of the 1 J couplings, 1 J YH , can be determined (note the 2h J NO cited previously [1]). In 1998, Dingley and Grzesiek [2] reported the first example of 2h J NN , which refers to 15 N– 15 N J couplings in solution. Using a uniformly 13 C/ 15 N-enriched nucleotide, they measured averaged values of 6.7 ± 0.5 Hz for uracil/adenine and 6.3 ± 0.2 Hz for cyto- sine/guanine dimers. Concurrently, Pervushin et al. [3] described similar results for DNA, corresponding to 6.3 ± 0.3 Hz for cyto- sine/guanine and 6.8 ± 0.3 Hz for thymine/adenine. Other exam- ples of biomolecules with observed coupling constants in this range have been reported [4] and we will cite only the case of the imidazolium/imidazole fragment in apomyoglobin, where a 2h J NN of 11.0 ± 1.0 Hz has been measured at pH = 4.9 [5]. In small molecules the 2h J NN couplings in solution, ranging from 16.5 Hz to 5.3 Hz, have primarily been determined for intramolec- ular hydrogen bonds [6,7]. The only contribution concerning inter- molecular hydrogen bonds is due to Sijbesma et al. dealing with ureidopyrimidinones that exhibit 2h J NN couplings with values of about 5 Hz that, in some cases appeared to be temperature depen- dent [8]. A previously published report by Del Bene and Bartlett calcu- lated a 2h J NN coupling for the CNHpyridine complex at 10.7 Hz, which was estimated from the Fermi contact term calculated with 1090-7807/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2010.07.015 * Corresponding author. Fax: +34 913988372. E-mail address: rclaramunt@ccia.uned.es (R.M. Claramunt). Journal of Magnetic Resonance 206 (2010) 274–279 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr