J. Phys. Chem. 1995,99, 9681-9686
9681
Correlation Times and Quadrupole Coupling Constants in Neat, Liquid Formamide R. Ludwig, J. Bohmann, and T. C. Farrar" Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received: February 7, 1995; In Final Form: April 14, 1995@
Nuclear magnetic resonance longitudinal relaxation rates of the dipolar nuclei I3C and I5N were measured in the liquid phase over the temperature range 261-395 K. Rotational correlation times were determined for the C-H and the N-H bonds and indicate that the motion is isotropic in the neat liquid. Longitudinal relaxation measurements for the quadrupolar nuclei 170, I4N, and 2H provide values of four different quadrupole coupling constants. The quadrupole coupling constants for the two amide deuterons are temperature dependent and vary from 230 to 290 kHz. The quadrupole coupling constant for oxygen ranges from 8.7 to 9.4 MHz. The quadrupole coupling constant for oxygen ranges from 8.7 to 9.4 MHz. The quadrupole coupling constants for the formyl deuteron (170 kHz) and the nitrogen-14 (2.84 MHz) are essentially temperature independent. Values for the quadrupole coupling constants provide information about the strength of the hydrogen bonds at different sites in the formamide molecule and their change with temperature.
1. Introduction Physical and chemical properties of formamide have been extensively investigated since formamide has been used for a long time as a solvent in chemical measurements and reactions. Being the smallest unit in a peptide chain, formamide is often selected as a model molecule in the study of breaking and fonning of peptide linkages occurring in biological systems. The structural and dynamic properties of formamide have been studied by ab initio calculation^,'-'^ molecular dynamics investigation^,'^^'^ different diffraction and scattering techniques such as X-ray,'0,'6-'9 n e ~ t r o n , ' ~ and - ~electron ~ diffraction,21,22 and depolarized R a ~ l e i g h m , ~i~c r ~ w a v e , ~and ~ - ~vibrational ~ s p e c t r o s ~ o p y , ~and ~ - dielectric ~~ relaxati~n.~~.~~ Nuclear magnetic resonance investigation^^^-^^ have also been reported. Most of the NMR studies are relaxation time or line width measurements for single nuclei at room temperature. None of this earlier work includes studies of liquid formamide for all nuclei andor for a large temperature range. 15N relaxation rates have been measured, but without an evaluation of the rotational correlation time.47 Other investigators studied quadrupole relaxation of 170,49 14N,48-49 and the formyl4*and amide deuterons;48two different strategies were used to evaluate the data. One strategy was to use literature values for the NMR correlation time of the C-H vector, z, obtained from similar molecules. A related strategy was to use dielectric relaxation data of neat formamide to obtain the Debye relaxation time, t ~and , then assume that the relation 32, = ZD was valid. Given the correct correlation time, one can then calculate the corresponding quadrupole coupling constant in formamide. The conversion of the collective dielectric relaxation times to a molecular correlation time is an especially dubious assumption. In a second strategy, values of the quadrupole coupling constants from gas or solid phase experiments were used to obtain the correlation times. From these results some investigators have concluded isotropic rotation and others have concluded anisotropic rotation. The reason for the ambiguity arises from the fact that the I4N quadrupole coupling constant for the solid phase (2.27 M H z ) and ~ ~ the gas phase (3.85 MHz)55,57s58 differ by about 75%. This introduces an uncertainty in the correlation
* Author @
to whom correspondence should be sent. Abstract published in Advance ACS Abstracts, June 1, 1995
time of a factor of 3. In addition, in almost all of the work done in the past, it has been assumed that the quadrupole coupling constants are independent of temperature. As seen below, this assumption is not always valid, and this introduces uncertainties in the interpretation of the experimental results. Consequently, on the basis of the work published in the scientific literature, it is still unclear whether liquid formamide rotates isotropically and the values of the quadrupole coupling constants in the liquid phase are still not known with much accuracy. The purpose of this investigation is to detennine more precisely the extent of any rotational anisotropy and to obtain accurate values for the quadrupole coupling constants in liquid formamide. As shown below, the values of the quadrupole coupling constants give important information about the strength of the hydrogen bonds at different sites in the formamide molecule and their change with temperature.
2. Experimental Section Spin-lattice relaxation times, T I ,of all nuclei in formamide were measured with the inversion-recovery pulse sequence using a Bruker AM 500 pulse spectrometer. The resonance frequencies were 125.72 MHz for I3C,76.75 MHz for 2H, 67.78 MHz for I7O, 50.66 for 15N, and 36.13 MHz for I4N, respectively. The nuclear Overhauser enhancements for I3C-'H and I5N'H were determined using simple gated decoupling experiments. The pulse sequence repetition rates used in all gated decoupling experiments exceeded 10 times T I . All experiments were done at natural abundance with the exception of the deuterium measurements. The I3C, I7O, I4N, and 2H relaxation times were measured in HC(O)ND2. I5N and 2H relaxation time measurements were done in DC(O)NH2. Both samples (C/D/N ISOTOPES, Canada) had a stated purity of more than 99 atom % D. They were degassed by several freeze-pump-thaw cycles and sealed under high vacuum ( Torr). 3. Results and Discussion a. Rotational Correlation Times. Formamide in the neat liquid at a temperature of 300 K or less is, essentially, a planar molecule (see Figure 3). The rotation about the C-N bond in this temperature range is highly hindered. This is shown by the fact that at 300 K in the high-resolution proton NMR
0022-3654/95/2099-9681$09.00/0 0 1995 American Chemical Society
Ludwig et al.
9682 J. Phys. Chem., Vol. 99, No. 24, 1995 0.5
0.4 c
h
I +
0.3
I
0.8
1
'0.2
0.4 0.2
0.1
0.0
0.0
3.5 4.0 1000/T/K-1 Figure 1. Carbon-13 (0)and nitrogen-15 (0)relaxation rates ( R I )of liquid formamide as a function of temperature. 2.5
TABLE 1: Experimental Relaxation Times, R1 (exp), Nuclear Overhauser Enhancements, 1, Dipolar Relaxation Rates, , Rotational Correlation Rl(DD), Bond Distances, m ~and Times, rc,for the C-H and the N-H Vector in Formamide at 298 K C-H N-H a
0.1190 0.0654
+1.71 -3.52
0.1018 0.0602
109.88" 100.15"
1 2.5
3.0
3.0
3.5
4.0
'I 000/ T/ K - 1 Figure 2. Carbon-13 (0)and nitrogen-15 (0)nuclear Overhauser effect in liquid formamide as a function of temperature.
and spin-rotation interactions and/or trace paramagnetic impurities. These small contributions may be taken into account by the fact that in the extreme narrowing region (WOZ,
