10928
J. Phys. Chem. 1996, 100, 10928-10935
Amidine N-C(N)-N Skeleton: Its Structure in Isolated and Hydrogen-Bonded Guanidines from ab Initio Calculations Ruggero Caminiti,* Andrea Pieretti, and Luigi Bencivenni Dipartimento di Chimica, Istituto Nazionale per la Fisica della Materia, UniVersita` “La Sapienza”, I-00185 Roma, Italy
Fabio Ramondo*,† Dipartimento di Chimica, Ingegneria Chimica e Materiali, UniVersita` dell’Aquila, I-67100 L’Aquila, Italy
Nico Sanna CASPUR c/o UniVersita` “La Sapienza”, I-00185 Roma, Italy ReceiVed: January 31, 1996X
Geometries of guanidine and eight of its N-imino derivatives (CH3, SiH3, OH, CN, F, Cl, CHO, and NO2) were calculated using ab initio molecular orbital techniques. MP2, MP4, and CISD geometries indicate that the guanidine molecule is pyramidal at amino groups and π-conjugation through the amidine skeleton is modest. MP2 structures of the eight N-imino guanidines reveal that substituting the hydrogen atom of the imino nitrogen by a functional group leads to a concerted variation of the CN bond distances. Topological electron density analyses indicate that the structural changes calculated upon N-imino substitution are largely due to changes of conjugation onto the amidine π-skeleton. Intermolecular hydrogen bonding involving the imino and amino groups is also found to affect the amidine geometry. Special attention has been paid in the work of 2-cyanoguanidine. The centrosymmetric dimer and a more extended hydrogen-bonded complex have been proposed as simulation of the crystal environment and the respective ab initio geometries (HF/ 4-31G(d) and MP2/6-31G(d)) are discussed. The structural changes due to self-association have been rationalized supposing that the contribution of polar canonical forms increases upon hydrogen-bonding formation.
Introduction The structure and properties of guanidine (HNdC(NH2)NH2) and its N-derivatives have been the subject of a wide number of experimental investigations.1 These compounds are special cases of n-π-conjugated heteroallylic systems. The extension of the conjugation system leads to a cross-conjugated or Y-delocalized hetero π-system, the amidine skeleton N-C(N)N, of the general type given by mesomeric forms shown in Figure 1. There is no experimentally determined geometry for the guanidine molecule available. Information on the structure of the isolated molecule has been obtained only from ab initio molecular orbital calculations.2,5 All the theoretical results2,5 indicate the CN bond to the imino NH group to be appreciably shorter than the two CN bonds to the amino NH2 groups. This structural feature is actually consistent with the canonical form I. The structure of the amidine functional group is instead known for numerous guanidine derivatives from solid state X-ray determinations.1 The amidine moiety in fact either may be included in cyclic systems or may exist in acyclic molecules having the general formula R-NdC[N(XX′)][N(X′′X′′′)] with R and X substituents attached to all allowed nitrogen atoms. Since experimental geometries for guanidine are lacking, information on its amidine skeleton could be obtained from those molecules having unsubstituted amino groups (X ) X′ ) X′′ ) X′′′ ) H). For this purpose, we focused our attention on † Address for correspondence: Department of Chemistry, Chemical Engineering and Material, Faculty of Sciences, Loc. Coppito, University of L’Aquila, I-67100 L’Aquila, Italy. Fax: 39-862-433753. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00311-5 CCC: $12.00
Figure 1. Canonical forms for guanidine.
five N-imino guanidines with the formula R-NdC(NH2)-NH2 having various R groups attached to the imino nitrogen.6,12 CN bond distances as obtained for the solid state molecule are given in Table 1. A scan of the experimental data indicates undoubtedly that substitution of the hydrogen atom of the imino nitrogen by a functional group leads to noticeable variations in the amidine skeleton. Its geometry is found substantially different from that predicted for unsubstituted guanidine, and in some cases, the three CN bonds even have similar lengths. Since the geometry of the N-C(N)-N functional group and particularly the equalization of the three CN bonds can be readily related to the extent of π-electron delocalization, it is worthwhile to investigate the origin of the differences between guanidine and its N-imino derivatives. However, comparing guanidine and its derivatives might be very complex since there are various factors that contribute to making their structures different. Firstly, we expect that N-imino substitution causes perturbations in the valence electron distribution of the molecule and, consequently, changes in the molecular geometry. Secondly, we should bear in mind that the experimental data of the guanidines reported in Table 1 are relative to the molecule in the crystal where intermolecular interactions may deform the geometry of the N-C(N)-N functional group. Last but not least, possible intramolecular interactions between © 1996 American Chemical Society
Amidine N-C(N)-N Skeleton
J. Phys. Chem., Vol. 100, No. 26, 1996 10929
TABLE 1: CN Bond Distancesa of the Amidine Fragment in Neutral Guanidines from Solid State Diffraction Studies (for Atomic Numbering, See Figure 2) b
2-nitroguanidine l-canavinec biguanidinec sulfaguanidine monohydratec 2-cyanoguanidined
CN(1)
CN(2)
1.309(7) 1.347 1.325(2) 1.334 1.3391(3)
1.359(9) 1.302 1.329(2) 1.348 1.3414(3)
CN(3)
ref
1.330(8) 6 1.361 8 1.361(2) 9 1.329 10 1.3327(3) 11
a The standard deviations in parentheses refer to the last significant digit. b Refinement by neutron powder diffraction. c X-ray single-crystal diffraction. d Low-temperature X-ray single-crystal diffraction.
Figure 3. Crystal structure of 2-cyanoguanidine.
properties of the isolated and hydrogen-bonded 2-cyanoguanidine are compared here to investigate the structural changes induced by intermolecular association. Computational Methods
Figure 2. Numbering of atoms in 2-cyanoguanidine.
the R substituent and amino groups might cause further distortions of the amidine geometry. The latter one is, for example, the case of nitroguanidine where the NO2 and NH2 groups interact through intramolecular O‚‚‚H-N hydrogen bonding.6,7 In some cases, all three above factors contribute to the deformation of the amidine skeleton of the guanidine molecule. The main aim of the present work is to investigate how each of the aforementioned factors affects the structure of the amidine functional group. The lack of any experimental result for neutral guanidines in the gas phase makes the ab initio molecular orbital methods an important tool to investigate the geometries of the isolated molecules. In order to gain a better understanding of the effects of the N-imino substitution, in section A we focus on eight N-imino guanidine derivatives with substituents, CH3, SiH3, OH, CN, F, Cl, CHO, and NO2, having different electron properties. The molecular structure and stability of the above guanidines were determined at different levels of ab initio theory, and the respective electronic structure is analyzed on the basis of the topological theory of atoms in molecules developed by Bader and co-workers.13,14 Section B deals with effects of intermolecular hydrogenbonding interactions on the guanidine skeleton. As a model system for such a study, we have chosen 2-cyanoguanidine (R ) CtN) (see Figure 2) since the structure of this molecule has been determined very accurately from low-temperature X-ray11 and neutron12 diffraction studies. Each 2-cyanoguanidine molecule in the crystal forms N‚‚‚H-N hydrogen bonds of two types by all five available protons, as shown in Figure 3. The first type connects monomers in cyclic dimers through the N(2)‚‚‚H(3)-N(3) hydrogen bonds, N(2)‚‚‚N(3) ) 2.993 Å. Each ring dimer is linked to CtN groups by a second type of hydrogen bonding, N-H‚‚‚NtC, weaker than the first type (N‚‚‚N ) 3.000-3.104 Å). Suitable molecular models simulating the neighboring intermolecular association in the 2-cyanoguanidine crystal are proposed here and studied by ab initio methods following the same theoretical approach successfully applied in recent studies on hydrogen bonding.15,19 Molecular
Ab initio molecular orbital calculations were run on an Alpha AXP-300/500 cluster of CASPUR c/o CICS at the University of Rome using the Gaussian 9420 package. Isolated Molecules. The geometries of the investigated guanidines were optimized at the Hartree-Fock (HF) and at the second order of the Moller-Plesset perturbation theory (MP2)21 employing the frozen core approximation (fc). The stationary points were determined without any symmetry constraint (C1 point group) by analytic gradient based techniques.22 Additional geometry optimizations were carried out for each molecule under the Cs symmetry constraint. The splitvalence basis sets 4-31G(d) and 6-31G(d) were used.23 The Cs and C1 symmetry geometries of unsubstituted guanidine were further calculated by application of the fourth order of the Moller-Plesset theory by including single, double, and quadruple excitations, MP4(SDQ)(fc)/6-31G(d), and by using the configuration interaction method including single and double excitations from the dominant configuration, CISD/6-31G(d). In the MP4 as well as CISD calculations, the core electrons were not included in the CI and geometry optimizations were carried out by CISD and MP4 gradient techniques as implemented in Gaussian 94. Atomic orbital population analysis was carried out using the natural bond orbital (NBO) method developed by Weinbold24 with the NBO subroutine of Gaussian 94. The electronic structure of the investigated guanidines was also analyzed by calculating the electron density distribution F(r), the gradient vector field of F(r), and its associated Laplacian as developed by Bader13,14 and co-workers using the subroutine implemented in Gaussian 94. The topological analyses were carried out by calculations at the MP2/6-31G(d) level employing all the active orbitals, MP2(full). Crystal Model of 2-Cyanoguanidine. Hydrogen bonding in 2-cyanoguanidine is investigated here by two molecular models. A centrosymmetric dimer consisting of two molecules hydrogen bonded pairwise by two N‚‚‚H-N bonds (Figure 4a) is firstly proposed. The geometry of dimer was fully optimized under the C2 symmetry constraint at the HF/4-31G(d) and MP2(fc)/6-31G(d) levels. A second model reproduced in Figure 4b was then introduced with the aim of simulating also the CtN‚‚‚H-N hydrogenbonding interactions occurring in the 2-cyanoguanidine crystal.
10930 J. Phys. Chem., Vol. 100, No. 26, 1996
Caminiti et al.
TABLE 2: Ab Initio Geometries of a and b Nonplanar Structures of Guanidine (See Figure 5) HF/6-31G(d) a
b
MP2(fc)/6-31G(d) a
b
MP4(SDQ)/6-31G(d) a
b
CISD/6-31G(d) a
b
CN(1) CN(2) CN(3)
1.387 1.260 1.385
1.377 1.262 1.385
Bond Length (Å) 1.402 1.395 1.286 1.287 1.398 1.399
1.405 1.282 1.401
1.399 1.283 1.402
1.394 1.271 1.391
1.387 1.272 1.392
N(1)H(1) N(1)H(1′) N(3)H(3) N(3)H(3′) N(2)H(2)
0.997 0.998 0.998 0.999 1.003
0.994 0.994 0.998 0.998 1.003
1.014 1.015 1.015 1.015 1.023
1.015 1.016 1.016 1.016 1.024
1.012 1.013 1.016 1.016 1.024
1.006 1.008 1.008 1.008 1.014
1.004 1.004 1.008 1.008 1.014
