CHEMICAL CLASSIFICATION AND H-BOND ENERGY EVALUATION FOR THE HETERONUCLEAR INTERMOLECULAR N-H...O BOND. COMPARISON WITH THE HOMONUCLEAR O-H...O CASE

Paola Gilli, Valerio Bertolasi, Valeria Ferretti and Gastone Gilli.

Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, University of Ferrara, Via Borsari 46, I44100 Ferrara, Italy.

Keywords: HYDROGEN BONDING, RAHB, CAHB, N-H...O BOND

It has been recently shown [1] that, in spite of the estraordinary variety of H-bonding situations normally observed, there can be only three possible classes of strong or very strong homonuclear O-H...O bonds, that is:

(1) -O-H...^-O= Negative Charge-Assisted H-Bonds, or (-)CAHB;

(2) =O...H⁺...O= Positive Charge-Assisted H-Bonds, or (+)CAHB;

(3) -O-H...O= where the two oxygens are connected by a ð-conjugated system, Resonance Assisted H-Bonds, or RAHB [2].

These findings have shown to be interpretable [1] in terms of a single H-bonding model, the Electrostatic-Covalent H-Bonding Model, that can be so briefly summarized:

(a) Weak H-bonds are electrostatic but become increasingly covalent with increasing strength;

(b) Very strong bonds are essentially three-centre-four-electron covalent bonds;

(c) Very strong H-bonds must be homonuclear (X-H...X) and symmetric because, in such a situation, the two VB resonance forms X-H...X and X...H-X are isoenergetic and can mix at the greatest extent;

(d) This last condition can be more generally expressed [5] as a condition of minimum ÄPA, that is of minimum difference between the proton affinities (PA) of the H-bond donor and acceptor atoms.

. The theory thus predicts that the heteronuclear N-H...O bonds must be systematically weaker than the homonuclear O-H...O ones and this prediction is tested here against more than 450 crystal structures forming the shortest N-H...O bonds.

H-Bond energies are calculated as E_HB = f [d(N...O), á(N-H-O)] by the Lippincott and Schroeder method [3] which is very quick, can account for non-linearity, and compares well with experimental and most recent ab initio energies. All energy calculations were accomplished by the program HBOND-LS written on purpose [4]

The results show that N-H...O bonds can be strengthened by RAHB and (+)CAHB, but not by (-)CAHB. Moreover, a new class of H-bonds appears that can be called (+/-)CAHB or ABHB (Acid-Base H-Bonds). In this class the H-bonds are related to the acid-base equilibrium

AcO-H...Nº _ AcO^-...H-N⁺º

and are of increasing strength with the decreasing value of ÄPA» Ä(pKa), the difference of PA (or pKa) between the AcO-H oxygenated acid and the Nº nitrogen base [5, 6].

The following comparison (see Table 1) of the intermolecular N-H...O and O-H...O geometries and corresponding H-bond energies confirms the relative weakness of the hetero-nuclear, compared to homonuclear, H-bond.

Table 1. Comparison of intermolecular N-H...O and O-H...O bonds (distances in Å and energies in kcal mol^-1; O-H...O data from Refs. 1 and 7).

[1] Gilli, Bertolasi, Ferretti and Gilli, J. Am. Chem. Soc., 116, 909 (1994).
[2] Gilli, Bellucci, Ferretti and Bertolasi, J. Am. Chem. Soc., 111, 1023 (1989)
[3] Lippincott and Schroeder, J. Chem. Phys., 23, 1099 (1955); J. Phys. Chem., 61, 921 (1957).
[4] Gilli and Gilli. HBOND-LS. A Program for Calculating H-Bond Physico-Chemical Properties According to Lippincott and Schroeder, University of Ferrara, Italy, 1992 (available on request). [5] Intermolecular Forces. Huyskens, Luck and Zeegers-Huyskens Eds., Springer, 1991.
[6] Malarski, Rospenk, Sobczyk and Grech, J. Phys. Chem., 86, 401 (1982).
[7] Bertolasi, Gilli, Ferretti and Gilli, Chem. Eur. J., 8, 925 (1996).