VARIABLE NMR SPIN-LATTICE RELAXATION-TIMES IN SECONDARY AMIDES - EFFECT OF RAMACHANDRAN ANGLES ON LIBRATIONAL DYNAMICS

Deuterium NMR spin-lattice relaxation times (T-1Z) of N-deuterated mic rocrystalline secondary amides vary from less than 1 s to more than 50 0 s at room temperature. The main motion effecting relaxation is an ou t-of-plane libration of the amide, as indicated by temperature-depende nt line shapes and anisotropic relaxation spectra. Over 25 amides were measured; they vary with respect to side chain sterics, hydrogen bond lengths, hydrogen bond geometry, and crystal packing, The temperature -dependent deuterium line shape and anisotropic relaxation rates indic ate an out-of-plane angular deflection of approximately 10 degrees; th e angle is probably similar for the rapidly and slowly relaxing amides , while the apparent time constant for the motion probably varies dram atically. Deuterons in methylene groups on both sides of the amide gro up for caprylolactam and caprolactam also indicate an out-of-plane lib ration with relaxation rates faster than that of the amide deuteron, p robably because the angular extent of the distortion is greater for th e flanking alpha-deuteron than for the amide deuteron. Carbon relaxati on measurements on lauryllactam indicate that the whole molecule libra tes to a comparable extent. Temperature-dependent relaxation rates for caprylolactam and caprolactam showed non-Arrhenius monotonic increase s in the relaxation rates with increasing temperature, as expected for libration dynamics; furthermore the quadrupolar relaxation measuremen ts support the assumption that the dominant spectral density contribut ion is above the Larmor frequency (i.e. T-1Q is longer than T-1Z). In aggregate, the data indicate that the motion effecting amide relaxatio n is a low-amplitude libration involving the entire molecule. Previous work on the librations of amides suggested that these librations are pronounced on the NMR time scale when the substance is near a phase tr ansition; we report here that there is additionally a relation between the extent of libration and the structure. Comparison of the relaxati on times to structures indicates that only amides with flanking alkyl groups on both sides (larger than a methyl group) exhibit extensive li bration; furthermore only those amides with both flanking dihedral ang les, phi {C2C1-NC(=O)} and psi {N(O=)C-C1'C-2'}, near -60 degrees (sim ilar to+/-40 degrees) have fast spin-lattice relaxation. On the other hand, correlation between the deuterium relaxation times and hydrogen bond length nor geometry nor crystal packing was observed. Variation i n the electronic structures of the conjugated amide groups was indirec tly probed by measuring the chemical shift anisotropy of the amide car bonyl carbon, the deuterium quadrupolar coupling constant, and vibrati onal frequencies. These parameters did not vary dramatically, indicati ng that the electronic structure is not strongly variable; the modest variation did not correlate with deuterium relaxation rates. The chemi cal shift tensor elements were delta(11) = 91.4 +/- 5, delta(22) = 185 +/- 8, and delta(33) = 245 +/- 3 ppm, the quadrupolar coupling consta nt and its anisotropy were 203 +/- 10 kHz and 0.15 +/- 0,02, the NH st retch frequency was 3300 +/- 42 cm(-1), and the carbonyl stretch frequ ency was 1644 +/- 25 cm(-1). We suggest a model in which the shape of the local potential associated with flanking alkyl groups leads to ''o verdamping'' of the amide Librational mode and generates slower (nanos econd) components in the vibrational frequency spectrum.