The observed and calculated oxygen‐17 quadrupolar coupling constant in nitrous oxide

Abstract

Nitrogen-14 and oxygen-17 NMR spin-lattice relaxation measurements on nitrous oxide yield a *'O nuclpar quadrupolar coupling constant of 12.4 If: 1.2 MHz. This result compares well with the value of 12.7 MHz obtained by scaling the value calculated using ab inifio molecular orbital theory at the 6-316* level.KEY WORDS Nitrous oxide, NMR TI, quadrupolar coupling constant, calculation of electric field gradients. INTRODUCTIONOxygen-17 nuclear quadrupolar coupling constants (QCCs) vary from less than 1MHz in the MOO:-anion' to a value in excess of 16MHz in Vaska's iridium compound truns-IrC1(CO)(PPh3)2.2 Although 170 nuclear quadrupolar coupling constants have been determined for a variety of diatomics and triatomics, a value for nitrous oxide (N,O) is still ~navailable.~ In this study I4N and 170 NMR spin-lattice relaxation (T,) measurements on N20 dissolved in n-hexane were used to obtain an experimental estimate of the 170 QCC in this relatively simple molecule. This result is compared with the value obtained using ub initio molecular orbital (MO) calculations at the 6-31G* level.4 ~~ EXPERIMENTALA sample of N20 (Matheson, 98%) was prepared by condensing the gas, at 77 K, into a thick-walled 10 mm NMR tube containing n-hexane (which had been previously degassed using three freeze-pump-thaw cycles). All manipulations were carried out on a vacuum gas-rack. To insure that the sample would not explode in the NMR probe the sample was first warmed in a shielded water-bath (30-35°C) for 15min. The sample tube did not burst under these conditions.All NMR measurements were performed on a Nicolet 360 NB spectrometer (B, = 8.48 T) using a broad-band variable-temperature 10 mm probe. The temperature was maintained with an accuracy of f 2 K by a nitrogen gas flow. Spin-lattice relaxation times were measured using the inversion recovery pulse sequence (IRCA) which uses compensating pulses.' The x/2 pulse widths were 50-6Ops. Eight scans at t Author to whom correspondence should be addressed. each delay in the IRCA experiment were carried out in the 14N relaxation experiments, while about 12 000 were required in the natural abundance I7O measurement. The resulting signal-to-noise ratio in the z = ~0 measurements was greater than 200: 1 for 14N and approximately 30: 1 for I7O. About 10 h of spectrometer time were required for the 170 q measurement. All & values were calculated from peak heights obtained at twelve or more different z values, using a non-linear three-parameter least-squares fitting procedure available on the Nicolet software.6 RESULTS AND DISCUSSIONBoth 14N and 170 are quadrupolar nuclei which are expected to relax entirely by the quadrupolar me~hanism.~ Under conditions of extreme narrowing the rate of relaxation of a quadrupolar nucleus of spin 1 is given by the equation7(1) where e2qQ/h is the nuclear quadrupolar coupling constant (QCC), q is an asymmetry parameter which must by symmetry be zero in a linear molecule and z2 is the rotational correlation time. Accurate values of the 14N QCC are available from gas...

Nuclear Quadrupole Coupling Constants for N2O: Experiment and Theory

“…Gas-Phase Nitrogen-14 Relaxation Data for N 2 O. The gas-phase NMR relaxation data given in Table 1 indicate that the 14 N T 1 values increase with increasing pressure (density) as expected. 66,67 Important is the observation that the average ratio, T 1 ( 14 N c )/T 1 ( 14 N t ), is 6.3 ± 0.3 independent of pressure within experimental error.…”

“…Very similar values were reported three years later by Reinartz et al 18 who found C Q ( 14 N t ) = −773.76 ± 0.27 kHz and C Q ( 14 N c ) = −267.58 ± 0.38 kHz yielding a ratio, [C Q ( 14 N t )/C Q ( 14 N c )] 2 , of 8.362. Clearly, the observed ratio of the 14 N T 1 values in n-hexane, 11.5 ± 0.7, is significantly greater than expected. In our 1986 paper, 14 we suggested that medium effects on the 14 N electricfield gradients (EFGs) were probably responsible for the T 1 ratio being greater than 8.3.…”

“…Using the gas-phase data obtained at a pressure of approximately 10 atm and solving the above two equations for a and b yields R 1 ( 14 N t ,SR) = 0.32 s −1 and R 1 ( 14 N t ,Q) = 23.15 s −1 (i.e., for the terminal nitrogen the spin-rotation relaxation mechanism contributes 1.4% to the total rate of relaxation, 23.47 s −1 ). For the central 14 N, R 1 ( 14 N c ,SR) = 0.89 s −1 and R 1 ( 14 N t ,Q) = 2.77 s −1 ; thus the spin-rotation mechanism contributes approximately 24% to the total rate of relaxation of the central nitrogen, 3.66 s −1 . Similar conclusions about the mechanism of nitrogen-14 relaxation for gaseous nitrous oxide have been previously been made by Jameson, ter Horst, and Jameson.…”

“… a NQCCs are determined using computed electric field gradients, and the values of the nuclear quadrupole moments from ref , i.e., Q N = 20.44 mb and Q O = −25.58 mb. b Reference . c Reference . d Reference for the (0, 0, 0) ground vibrational state. e Reference for the (0, 1 1 , 0) first excited bending vibrational state; f Reference . Note that the results were originally reported used scaled conversion factors going from the EFG to the NQCC.…”

“…Clearly, the observed ratio of the 14 N T 1 values in n -hexane, 11.5 ± 0.7, is significantly greater than expected. In our 1986 paper, we suggested that medium effects on the 14 N electric-field gradients (EFGs) were probably responsible for the T 1 ratio being greater than 8.3. Because the nuclear quadrupole coupling constant C Q for the central 14 N is very small, we suspected that it might be more sensitive to medium effects than the terminal nitrogen.…”