Degrees of freedom effect and internal energy partitioning upon ion decomposition

The effect of ion size on rate of dissociation: RRKM calculations on model large polypeptide ions

“…According to our RRKM calculations, as ions get larger, their rates of dissociation decrease at a given internal energy. This is in accord with previous experimental and theoretical results [7][8][9][10][11]. Also, as in previous work, the effect of size on the rates is offset somewhat by parallel increases in thermal energy content.…”

“…where G*( E -Eo) is the sum of the number of states in the transition state at E -Eo, p( E) is the density of states in the reactant ion at energy E, (7' is the reaction path degeneracy, and h is Planck's constant. Early detailed RRKM calculations and experiment [9][10][11] indicated that, as precursor ions become larger, on average the fraction of available energy deposited in a product ion becomes smaller, making it less reactive, even when thermal energy was included in the energy content of the ion. The RRK expression does not reliably make quantitative predictions, as v is usually treated as an adjustable parameter [12].…”

Transfer of vibrational energy from highly excited butyl radicals. Relative collision diameters of homologous n-perfluoroalkane bath molecules

“…The Journal of Physical Chemistry, Vol. 6 Obtained using dcCCH») = 0.60, ^(CF*) = 0.85, and /3C(M) = 1.00 for C2 and higher homologa to determine sbm, together with the Lennard-Jones values for ßµ22*( *) at 298°K (Appendix I) to obtain • c Obtained using the value of /3C(M) determined as in footnote h, Table , together with the value of ßµ22* (Appendix I) to obtain tbm• d Reference 9.e Extrapolated from higher homologs in this series.f L. W. Flynn and G. Thodos, AIChEJ., 8,362 (1962 " /3C(M) values determined from eq 6 using the Lennard-Jones ratio of ßµ22* integrals and /3C(M) = 1 at 195°K. 6 flc(M) values determined from eq 6 using a corrected value for ßµ22* at 195°K such that the ratio of ßµ22* integrals makes /3c(C<Fxo) equal to 1.00 at both temperatures; this ratio is the parenthetic quantity in the adjoining column and is, of course, also the resulting value of the s2 ratio.c Based on theoretical .…”

“…The reference collision diameter at 195°K, sbb for the butyl-butene pair, was assigned a value of 7.50 Á, based on a Lennard-Jones extrapolation from room temperature, sbb195 = 8ßß298[ ßß22*(195)/ ßß22*(298)]1/2, assumed valid for hydrocarbons. The requirement that the Lennard-Jones predicted collision diameter should equal the experimental quantity for these species leads to the following relation between /3C(M) at 298 and at 195°K, ß02 9 8( )/ß/95( ) = (W-Ri95)(nBM22(195)/ ßµ22 (298))/ ( ßß22 (195)/ ßß22 ( 298)) (6) where R29& -slopeeM/slopeBB at 298°K, and R195 = slopeBM/slopeaB at 195°K , and are the experimental quantities measured at the two temperatures. The ratio of integrals for BM and BB give the temperature dependence of sbm and sbb, respectively.…”

Ion decomposition versus molecular size probed by vacuum ultraviolet photoionization

“…Given the large number of potential variables, it is not surprising that consistent, assignable trends in primary fragmentation with increasing molecular size have not been reported. Consistent trends in secondary fragmentation with increasing molecular size, however, are routinely observed and give rise to the so-ctll!ed degrees-ot-freedom effect [5][6][7][8]. Secondary fragmentation is apparently easier to study because the effects of most of the variables are either eliminated or I f averaged out" by the primary fragmentation step [9].…”