J . Phys. Chem. 1985,89, 1105-1 107 salts to understand better the role of various effects that govern the conformational stability of a protein in aqueous neutral salt solutions. Registry No. NaCI, 7647-14-5;glycine, 56-40-6; diglycine, 142-73-4; triglycine, 139-13-9; DL-alanine, 302-72-7; DL-a-aminobutyric acid, 2835-81-6; L-valine, 72-1 8-4; L-leucine, 61-90-5; glycyl-DL-alanine,92677-2; glycyl-DL-a-aminobutyric acid, 7369-76-8;glycyl-L-valine, 1963-
A Reexamination of the CH,' Isotope Effect
1105
21-9; glycyl-L-leucine, 869-19-2; L-leucylglycine, 686-50-0.
Supplementary Material Available: Data on apparent molar volumes (4") and apparent molar heat capacities (&) for some amino acids and peptides at 298.15 K in aqueous solutions of Sodium chloride as a function of concentration of sodium chloride given in Table I (7 pages). Ordering information is available on any current masthead page.
-t HCN Association Reaction Including the CH,+/CD,+
Paul R. Kemper, Lewis M. Bass,+ and Michael T. Bowers* Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: August 1, 1984; In Final Form: October 22, 1984)
--
The association reaction CH3++ HCN [CH3-HCN]+ has been reexamined. Previous work suggested this reaction occurred with a bimolecular rate constant of 1 X lo-'' cm3/s due to radiative association at temperatures near 300 K. New work reported here suggests an upper limit of 5 X lo-'' cm3/s for radiative association at 300 K. Third-order association rate constants are reported for He, Ne, Ar, and HCN third bodies. Relative stabilization efficiencies of 0.11, 0.23, 0.25, and 1.OO,respectively, are obtained. The effect of substituting CD3+for CH3+ was also studied. The third-order rate constants increased by approximately a factor of 2, in excellent agreement with phase space theory calculations. The reaction CD3+ + HCN CD2H+ DCN was observed to occur at approximately 15%of the collision rate, indicating a barrier to isomerization exists with a height comparable to the [CD3-HCN]+ dissociation energy.
-
+
Introduction Several years ago we reported' an investigation, both experimental and theoretical, of the CH3++ H C N association reaction
The experiments were done using a drift ion cyclotron resonance spectrometer (drift ICR). Experimental evidence indicated that the stable product was produced both by radiative and collisional stabilization of an excited association complex. Two experimental findings indicated that radiative stabilization occurred: first, the observed second-order reaction rate coefficient did not extrapolate to zero at zero pressure. This indicated that stable product was formed in the absence of collisions. Second, ion cyclotron double-resonance experiments indicated that the CH3+reactant and the [CH3-HCN]+ product were chemically coupled at low pressure. In this paper we present new measurements of both the CH3+ + H C N and the CD3++ H C N systems. The experiments were done using both drift and tandem ICR spectrometers. We now conclude that radiative stabilization does not occur to a significant extent in these systems. We do, however, find very significant differences between the third-order association rates in the CH3+ and CD3+ systems. These differences are well reproduced in theoretical calculations using statistical theory.
Experimental Section Both drift ICR and tandem ICR (TICR) spectrometers were used in the experiments reported here. Both spectrometers have The drift ICR experiments can be been described previou~ly.~-~ briefly described as follows (see also ref 1). The CH3+reactant was formed by low-energy electron impact on CHJ. The resulting CH31+ (>90% of the ionization) was ejected throughout the cell by using ICR double resonance. This was necessary since the CH,I+ ion also reacted with H C N to give a mass 42+ product (the product of interest)
CH31+ + H C N
-
[CH3-HCN]+
+I
(2) In the previous study, we obtained the observed second-order rate from the relative CH3+and M42+ intensities knowing the reaction time, H C N pressure, and bath gas pressure. The same experimental conditions were used in the present experiments; however, the drift ICR was used only for low-pressure, thermal energy ICR double-resonance experiments and no rate constants were measured. In these doubleresonance experiments, the M42' product ion was monitored in the second region of the cell while the reactant ion of interest was irradiated at its resonant cyclotron frequency in the first region of the cell. The resulting increase in reaction kinetic energy allowed determination of the effect of kinetic energy in the reaction, as well as the different reaction pathways which lead to the M42+ product. The buik of the experiments presented here were done using the TICR spectrometer. In the TICR the ion formation and reaction regions are separated and interfering reactions (such as the CH31+ H C N reaction) do not occur. Ions are formed in an ion source by electron impact. Both CH4 and CH31were used as sources of CH3+ ions; no difference in the results was found. After formation the ions were drifted out of the source, accelerated to -3 KeV, mass selected in a 180' Dempster mass filter, decelerated to near zero kinetic energy, and injected into the ICR reaction cell where the reaction with H C N occurred. The H C N used was purified by freeze-pump-thaw cycles. The source and reaction cell pressures were measured directly with a capacitance manometer. Reaction drift times were measured by using a trapping pulse method.3 The CH3+ kinetic energy upon injection into the reaction cell is about 0.5 eV (lab).3 This excess energy could have a significant effect on the association reaction. The effect-of-this injection energy was investigated by adding nonreactive collisional buffer gases to the reaction cell. This point
+
(1) L. M. Bass, P. R. Kemper, V. G . Anicich, and M. T. Bowers, J . Am. Chem. Soc., 103, 5283 (1981). (2) A. G . Wren, P. Gilbert, and M. T. Bowers, Rea Sci. Inrtrum., 49, 531 (1978). '
Present address: Division of Natural Sciences, University of South Carolina at Aiken, Aiken, SC 29801.
0022-365418512089-1105$01 SO10
(3jP. R. Kemper and M. T. Bowers, Int. J . Mass. Spectrom. Ion Phys., 52,
l(1983).
0 1985 American Chemical Society
Kemper et al.
1106 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1
1
I
A
1
TABLE I: Third-Order Rate Coefficients for CH3'/CD3+ Reaction stabilization 1 0 ~ ~ k ,cm6/s ,~,
CH,+
gas
He
W
Ne E " W n o E
A
= HCN
Ar
LL
HCN
00
CD,+
2.2 2.1" 3.5 3.0" 6.2 1.07 X IO2
" Previous value, ref
+ HCN ratio
4.4
2.00
7.5
2.15
12.0 2.2 X IO2
1.94 2.05
1.
TABLE 11: Relative Stabilization Efficiencies for Different Bath Gases
stabilization gas (M) He Ne
Ar HCN
PdPHCN
109k,," cm3/s
0.11 0.23 0.25
0.55 0.40 0.66 2.63
.oo
1
" Calculated from ADO8 or Langevin theory.' PRESSURE OF BATH GAS, x 10-3 TORR
figure 1. Plot of k;?
+
vs. bath gas pressure for the CH3+ HCN system.
is discussed later. ICR double resonance was also used in these TICR experiments to determine the effect of additional CH3+ kinetic energy on the association reaction. Again, the M42+ product was monitored in the second region of the reaction cell while the CH3+ions were irradiated in the first region, increasing their kinetic energy. The rate constants reported here were determined by measuring the CH3+and M42+ intensities as a function of buffer gas pressure with a small, fixed H C N pressure. We calculated the observed second-order rate constant knowing the ion intensities, the H C N concentration, and the reaction time. The second-order rate was plotted vs. buffer gas pressure, and the third-order reaction rate constant obtained from the slope. This analysis is discussed in the next section.
Results and Discussion Rate Coefficients. A kinetic scheme appropriate to reaction 1 was developed in ref 1.
This scheme assumes both radiative and collisional stabilization occur; k f is the formation rate coefficient, kb is the unimolecular back-dissociation rate coefficient, k, is the radiative rate, k, is the rate coefficient for collisions between the complex and the bath gas M, and P is the collisional stabilization efficiency (0 I/3 I 1.O). The protonated acetonitrile structure of the stable association product was determined by collision-induced dissociation mass spectrometry .4 If a steady-state analysis is done on reaction 3, the following expression for the observed second-order and third-order rate coefficients for reaction 1 are obtained'
A plot of ki:f vs. bath gas pressure is shown in Figure 1 for the CH3+ H C N system with He, Ne, Ar, and HCN bath gases. It is clear from the different slopes that k3,d(HCN) >> ~k,,~(Ar) > k,,(Ne) > k,,,(He). The actual values of k3,d for CH3+and CD3+ with the different bath gases are listed in Table I. The present values of k3d for CH3+ H C N in He and Ne agree fairly well with those measured previously.' Note the large increase in k3rdwhen H C N is used as the bath gas. Since the increase in k, is modest, this must reflect a large increase in collisional stabilization efficiency through such mechanisms as H C N ligand switching and direct v-v transfer. The stabilization efficiencies relative to HCN (P(X)/P(HCN)) for the different bath gases are listed in Table 11. The absence of significant radiative stabilization in this system can be readily inferred from Figure 1. Note that all the zeropressure values of kS:id are near zero or slightly negative. If a radiative channel which leads to a low-pressure secondary rate 1 X 1O-Io cm3/s were present, as we proposed coefficient of cm3/s previously, then a zero-pressure intercept of 1 X should be present. On the basis of Figure 1, we estimate that any bimolecular association must proceed with a rate coefficient of
