(英文)温度和压强对CH3CF3和CD3CF3的氟谱化学位移的影响

EXPERIMENTAL AND RESULTS
We were primarily interested in the study of the second virial screening coefficient ol. The screening constant u is directly related to IJ~by the following equation (2): u - cro= u,/v, $ u,/v,; + * * *, PII
u--o=-
01
V?ll
and
U, =M--.
a6
aP
Here, M is the molecular weight, p the gas density in gm/cc, the screening constant for an isolated unperturbed molecule is uo, and 6 is the chemical shift. From this relationship, the temperature dependence for both (Tand ul are obtained. The density and temperature dependences of the 19F chemical shifts for CH,CF, and CD,CF, gases are shown in Fig. 1. Chemical shifts were measured in the pressure range 5-25 atm. Below 5 atm, the resonance signal was not strong enough to measure by conventional methods. Above 25 atm, the gas condensed. Since the sensitivity decreases by the Boltzmann factor and there is a chance of explosion at higher temperatures, measurement above 77°C was not taken. The 19F chemical shifts at zero pressure and room temperature are given in Table 1 and the second virial screening coefficients are reported in Table 2. Experimental chemical shift data give a,(CH,CF,) - u,(CD,CF,) = (-0.284 & 0.018) x 1O-6
JOURNAL
OF MAGNETIC
RESONANCE
8,152-l
58 (1972)
Temperature and Pressure Dependence of 19FChemical ~~if~~ i Pure CH,CF, and CD,CF, Gases*
S. M~HANTY
Columbia University, New York 10027
In a recent article (I), we have reported the temperature and pressure dependence of chemical shifts in pure CF4, SiF, and SF, gases and in gaseous mixtures The results were interpreted in terms of intermolecular contributions (I, 2). Any vibrational contribution to the observed temperature dependence was assumed to be small and neglected. A qualitative discussion justified this assumption for these systems. Ramsey (3) and Buckingham (4) have given theories for vibrational and rotational effects on the shielding constant. Hindermann and Cornwell (5) have attributed the measured difference in r9F shielding between DF and HF molecules in the gas phase to zeropoint vibrational averaging. Raynes et al. (6) have made vibrational and rotational averaging for the ‘H shift between HD and Hz molecules. Petrakis and Sederholm (7) first reported the temperature dependence of ‘H chemical shift in gases and ascribed this effect to excitation of vibrational modes of the molecules. In most of these works, the intermolecular contribution has been neglected completely. The experimentally observed temperature dependence of the chemical shift of any gaseous system is the sum of intermolecular and intramolecular (vibrational and rotational excitations) contributions. Buckingham (4) has discussed all these effects for rH shifts in HCl gas. For diatomic isotopic molecules like HF and DF, the “rotational” contributions to u and dajdT are the same (4). Only excitation of vibrational modes contributes to the change in the chemical shifts in such isotopic molecules so far as the intramolecular effect is concerned. The vibrational effect is expected to be large for vibrational modes of low frequency. Consequently, we have selected two isotopic molecules CH,CF3 and CD3CF3 with low vibrational (e.g., torsional) frequencies.
where V, is the molar volume. Since ‘H and 19F chemical shifts are found experimentally to be linear with density of the gas, higher-order corrections can be neglected. Thus, one has
AND

H. J.
BERNSTEIN
Division of Chemistry, National Research Council of Canada, Ottawa, Ontario KlA OR6, Canada Received December 3,197l Precise 19F NMR chemical shifts of CH3CF3 and CD&Z&in the gas phase have been obtained. These shifts are linear with density and nonlinear with temperature. The temperature dependence is described in terms of intermolecular and intramolecular (vibrational and rotational excitations) contributions to shielding. Relative magnitudes of these contributions are discussed. INTRODUCTION
IgF
* Issued as NRCC No. 12590.
Copyright Q 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.
152
FLUORINE
CHEMICAL
SHIFTS
IN
GASES
at 30°C and (-0.285 i 0.018) x low6 at 77°C. These shielding differences are equal within the experimental errors and quite small compared to their absolute values (-263 x lo+). At constant pressure (10 atm), lgF chemical shifts relative to the gas at zero pressure are For CH,CF,, 0.360 x 10m6at 30°C