Anharmonic effects on theoretical IR line shapes of H-bonds

In the spirit of Y. Marechal and A. Witkowski's [J. Chem. Phys. 48 (1968) 2 697] work, one revisits, for weak H-bonds, the dependence of the angular fr equency omega and of the equilibrium position q(e) of the upsilon(X-H) high frequency mode q, on the position coordinate Q of the low frequency upsilo n(X-H ... Y) mode. One considers: omega = omega degrees + bQ + cQ(2) and q( e) = gQ + fQ(2). That leads to the anharmonic potential U: U = k(1)q(2) + S igma k(r)Q(r) + Sigma Sigma k(nm)q(n)Q(m) + k(15)qQ(5). Here k(r) and k(mn) (r = 2-5, n = 1,2 and m = 1-4) are interrelated through b, c, g and f. By aid of the Hamiltonian involving U, we find the direct damped auto correlat ion function of upsilon(X-H), which, by Fourier transform, gives the IR spe ctral density (SD). When only b not equal 0, the SD is nothing but that giv en in a previous paper [P. Blaise, O. Henri-Kousseau, Chem. Phys. 243 (1999 ) 229]. When the adiabatic approximation is performed, this SD becomes that of N. Rosch and M. Ratner [J. Chem. Phys. 61 (1974) 3444] which reduces in rum to that of Marechal and Witkowski in the absence of damping. With resp ect to b not equal 0, c produces a narrowing of the SD if c > 0 and a subtl e broadening if c < 0. Besides, g induces the same narrowing for g > 0 and g < 0, while f gives subtle changes very sensitive to the sign of f and to the values of b, c and g. The situation b < 0 and f > 0 which is physically the most probable, leads to SDs which are the most evoking experimental pr ofiles. (C) 1999 Elsevier Science B.V. All rights reserved.