Unimolecular dissociation of polyatomic ions by molecular beam photoionization mass spectrometry and collision-induced dissociation

Abstract

<p>The photoionization efficiency (PIE) specta for M(CO)n+ (n = 0-6) from M(CO)6, M = Cr, Mo, and W, have been measured in the photon energy range of 650-1600 A. Based on the ionization energies for M(CO)6 and appearance energies (AEs) for M(CO)n+ (n = 0-5) determined here, we have obtained estimates for the sequential bond dissociation energies (D0) for CO-M(CO)n-1+ (n = 1-6). The comparison of the first D0 values for M(CO)6+ obtained here and those for M(CO)6 provides strong support for the theoretical analysis that the importance of relativistic effects, which give rise to more efficient M to CO [pi]-back-donation in M(CO)6, is in the order W(CO)6>Mo(CO)6>Cr(CO)6.;The appearance energies for the formation of SF3+,sF4+, and SF5+ from SF6, and SF2+ and SF3+ from SF4 are reexamined by the molecular beam photoionization mass spectrometric method. Based on comparisons between G2 and G2(MP2) predictions and experimental measurements, we recommend a self-consistent set of experimental [delta]fH°0 for SFn,sFn+, and SFn- (n = 1-6). We have rationalized the theoretical structures for SFn,sFn+, and SFn- (n = 1-6) using the valence-shell-electron-pair-repulsion theory. The alternating patterns of high and low values observed for the SFn-1-F (n = 2-6), SFn-1+-F (n = 3-5), and SFn-1--F (n = 2-6) bond dissociation energies at 0 K and for the IEs and EAs of SFn (n = 1-6) are attributed to special stabilities for closed-shell molecular species with fully-filled valence electron shells around the central S atoms;Strong preference is observed for the C-S bond scission process, leading to the formation of CH3+ + SH, CH3CH2+ + SH, and CH2SH+ + CH3 in the collision induced dissociation (CID) reaction of CH3SH+, CH3CH2SH+, and CH3SCH3+ + Ar, respectively. Since the dissociation energies for C-S bond are significantly higher than that of H-elimination, this observation indicates that the CID process is non-statistical. The high yield for the C-S bond breakage is attributed to the more efficient translational to vibrational energy transfer for the C-S stretching mode than for C-H and S-H stretching modes via collisional activation, and to weak couplings between the low frequency C-S and high frequency C-H and S-H stretching vibrational modes of CH3SH+, CH3CH2SH+ and CH3SCH3+.</p>