We have also measured the 13C Isotropic shifts and line widths for the cobalt complexes of glycine and alanine with a 13C NMR spectrometer and obtained a pD dependence of the shift and line width similar to the case for histidine. (29) For Co2+ complexes, the experiment values of the electron spin relaxation time rs are in the range 10"12-10"13 s30 at room temperature while the rotational correlation time rr at the same temperature Is approximately 10-10-10-11 s31 for molecules of the size of a histidine-cobalt complex In solution. Therefore, under these conditions the relation 1/rs » 1/rr holds. Furthermore, we obtain the following relations In the case of very fast electron spin relaxation time: t, » tm"\ rc = ra = ts. Using these effective correlation times in combination with the relations, ^ ,2 » 1, =2 2 » 1, the Solomon-Bloembergen equations are reduced to32-34 the pressure variation of this ratio with model calculations, the average vibrational energy, (AE)¿own, removed from CH3CF3 per collision was assigned for each of the bath gases. For monatomic, diatomic, and triatomic gases, the (AE}down values ranged from 1 to 2 kcal moT1 and the distribution of transition probabilities has an exponential dependence on AE. For larger polyatomic bath gases, a Gaussian distribution, represented here by a stepladder model, provides a satisfactory description of the transition probability distribution and (AE}down increases to a maximum value of ~10 kcal moT1 for the most efficient bath gases. The present data, together with results already in the literature, provide a rather complete description for the vibrational deactivation of chemically activated CH3CF3*. Vibrational energy transfer from CH3CF3* occurs somewhat less readily than from many other chemically activated molecules, such as 1,2-dichloroethane, cyclopropane, or alkyl radicals.

No abstract

We wish to communicate results from studies of the vibrational deactivation of chemically activated molecules [ 11. The present measurements are for CHaCFZ with a wide variety of bath gases which include inefficient rare gases as well as polyatomic gases ranging up to the efficiency of those investigated by Chang, Craig, and Setser [2]. The results consist of comparisons of data at high pressure, which give a measure of relative collisional deactivation efficiencies, and of extensive low-pressure data, which permit approximate assignment of a collisional transition probability model and the average energy ( A E ) lost per collision according to that model. For several bath gases experiments were done at 300°K and 195°K. The important general conclusions are (a) there is a wide spread in collisional efficiencies for various bath gases, and (b) CH 3CF 3 is particularly difficult to deactivate relative to 1,2-dichloroethane [3] and cyclopropane [4].The photolysis [5] of CF3N2CH3 generates CH3 and CF3 radicals from which chemically activated CHBCF3" molecules are formed, with an average energy of 102 kcal/rnole [Z]. The critical energy for H F elimination producing CHz=CF2 is -68 kcal/mole [6]. Chemically activated CH3CF3* may either undergo unimolecular reaction or collisional stabilization. The nonequilibrium rate constant for this process is defined by k, = wD/S, where D is the number of molecules decomposing per unit time, S is the number of molecules stabilized per unit time, and w is the collision frequency. The proportionality between w and pressure allows the rate constant to be initially recorded as k, = P(D/S); conversion to sec-' units was done in the usual manner. The experiments consisted of mapping the increase in k, with decreasing pressure by gas chromatographic measurement of D / S over a wide pressure range. For a particular bath gas the mole ratio of bath gas to CH 3N2CF 3 was constant and sufficiently high that collisions with CH3N2CF3 could be neglected. The experimental results for SFs, CFr, CH4, and N P are presented in Figure 1. The limiting high-pressure rate constants k,", the standard deviations, and the "best fit" curves were cjbtained by polynomial regression analysis. Table I presents a summary of the results.The competition between unimolecular reaction and cascade deactivation for the i t h energy increment can be represented by the master equation [1,2,4]. Computational solution of the master equation to obtain steady-state concentra- 473

Articles you may be interested inMeasured radiative lifetimes of rovibronic levels in the A1Π(v=0) state of CO and comparison with theory AIP Conf.Radiative decay rates from deperturbed v=0-7 vibrational levels of CO A 1Π measured using synchrotron radiation Th decay of the CO( a 3I1, v' = 0-3) levels have been studied in a flowing-afterglow apparatus. The e 3 X 1,<,+) . . . t 't Th results vibrational distribution was monitored by observing the CO(a I1-""'. emtSS10n In ensl y. e , were interpreted to obtain 52.5, 77, and 178 sec-1 for the infrared radiative dec~y.constants for the v = I, 2, and 3 vibrational levels of CO( a 3I1). The data also establish that the upper limtt to t~e rate cons~t for vibrational relaxation ofCO(a) by collision with He is :<;;6XIO-17 cm 3 moiecule-1 sec-, corresponding to a collision probability of :<;; 1.5 X 10-7 .

Publication costs assisted by the Department of DefenseNonequilibrium vibrational distributions of CO+(A2n,u'=0-6), CS(A1rI,u'=&5), and Cz(A3TI,u '=M) were prepared by collisional processes in a 300-I< helium flowing-afterglow apparatus. The vibrational band intensities of the electronic emission systems were used to obtain the steady-state vibrational distributions from 0.8 to 15 torr. Extensive vibrational relaxation by collisions with He was observed for CO"(A) and CS(A), but not for C,(A), over this pressure range, Electronic quenching of CS(A) probably is competitive with vibrational relaxation, even in helium. The data were fitted to relaxation models based upon Au = 1 collisional transitions by using the steady-state master equation formulation. The Au = 1 relaxation cross sections for CO+(A) and CS(A) with He are in the range of 0.01 of the gas kinetic values. The upper limit to the 4 u = -1 relaxation cross section for C,(A) is 5 X of the gas kinetic value. Studies of the relaxation of CS(A'n) in Ar were attempted, but electronic quenching appeared to dominate over vibrational relaxation. These results are compared to vibrational-translational relaxation of other electronically excited states.

No abstract