Thermodynamics and Mechanisms for Decomposition of Protonated

ARTICLE pubs.acs.org/JPCA

Thermodynamics and Mechanisms for Decomposition of Protonated Glycine and Its Protonated Dimer P. B. Armentrout,* A. L. Heaton,† and S. J. Ye‡ Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States

bS Supporting Information ABSTRACT: We present a full molecular description of fragmentation reactions of protonated glycine (G) and its protonated dimer, HþG2, by studying their collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). In contrast to previous results, it is clear that HþG decomposes by loss of CO followed by H2O. Analysis of the energy-dependent CID cross sections provides the 0 K barriers for these processes as well as for the binding energy of the dimer after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion
molecule collisions. Relaxed potential energy surface scans performed at the B3LYP/6-31G(d) level are used to map the reaction surfaces and identify the transition states (TSs) and intermediate reaction species for the reactions, structures that are further optimized at the B3LYP/6311þG(d,p) level. Single-point energies of the key optimized structures are calculated at B3LYP and MP2(full) levels using a 6-311þG(2d,2p) basis set. These theoretical results are compared to extensive calculations in the literature and to the experimental energies. The combination of both experimental work and quantum chemical calculations allows for a complete characterization of the elementary steps of HþG and HþG2 decomposition. These results make it clear that HþG is the simplest model for the ‘‘mobile proton’’, a key concept in understanding the fragmentation of protonated proteins.

’ INTRODUCTION One of the primary decomposition pathways available to protonated amino acids and peptides is the loss of [C,2H,2O], a process investigated in numerous gas-phase studies.1
4 In the simplest protonated amino acid, HþG, this fragmentation is the only lowenergy pathway available. Thus, this simple system provides a venue for detailed examination of the process without complicating decomposition pathways. Quantum chemical theory has been used to characterize multiple pathways for this process,5 finding that the most thermodynamically and kinetically favored mechanism involves loss of the combined elements (H2O þ CO). Despite the importance of this decomposition in understanding gas-phase reactivity of biologically relevant amino acids, quantitative experimental characterizations of this process are limited.1
4,6 For instance, the only experimental information available for fragmentation of HþG comes from the early pioneering study of Klassen and Kebarle6 (KK). They measured the dominant, low-energy collision-induced dissociation (CID) fragment ions of protonated Gn (n = 1
4) and several related compounds using a modified triple quadrupole mass spectrometer. Wesdemiotis and co-workers have examined the metastable and collision-induced decomposition of HþG as well as its neutralization
reionization behavior.7 Significant progress in analyzing such processes theoretically has been made over the past decade. Decomposition of HþG has been explored by KK,6 Uggerud,8 and Balta et al.,9 with a subsequent study by O’Hair et al. including a much more comprehensive exploration of the potential energy surfaces.5 r 2011 American Chemical Society

The protonated dimer of glycine has been evaluated thoroughly. First studied by Meot-Ner et al. using high-pressure mass spectrometry (HPMS), the GHþ
G bond was measured as 130 kJ/mol.10 Williams and co-workers used blackbody infrared dissociation (BIRD) to measure the HþG2 binding energy as 114 ( 5 kJ/mol from master equation modeling of the temperature-dependent dissociation rates.11 These authors also performed theoretical work to determine the ground state conformer. Lower-energy structures were later found by Raspopov and McMahon,12 who also measured the binding energy using pulsed-ionization high-pressure mass spectrometry (PHPMS), finding 114 ( 7 kJ/mol, in good agreement with the BIRD measurement. The dimer has also been examined using infrared photodissociation spectroscopies in both the 3050
3800 cm
1 and 700
2000 cm
1 range in comparison with quantum chemical calculations.13
15 The most recent work agrees that the ground state structure is most consistent with a proton shared between the amine group on one glycine and the carbonyl of another,14,16 as discussed further below. Here, we comprehensively characterize the fragmentation reactions of protonated glycine (HþG) and its proton-bound Special Issue: Pavel Hobza Festschrift Received: March 18, 2011 Revised: April 7, 2011 Published: April 20, 2011 11144

dx.doi.org/10.1021/jp2025939 | J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A dimer, HþG2, using gas-phase threshold collision-induced dissociation (TCID) experiments carried out in a guided ion beam tandem mass spectrometer (GIBMS). We measure absolute experimental energetics for the observed reactions and use theoretical calculations at the B3LYP/6-311þG(d,p) level to provide structures, vibrational frequencies, and rotational constants needed for accurate analysis. Experimental threshold energies are compared to single-point calculations performed at the B3LYP and MP2(full) levels using a 6-311þG(2d,2p) basis set to fully characterize the key steps of HþG and HþG2 decomposition. For HþG, new experimental thermochemistry is obtained that clarifies the mechanism for fragmentation previously elucidated5,8,17
19 and verified here at higher levels of theory. These results also make it clear that HþG is the simplest model for the ‘‘mobile proton’’,20 a key concept in understanding the fragmentation of protonated proteins. The dimer bond energy determined here agrees well with previous equilibrium determinations,11,12 with a new low-lying conformation identified theoretically. Overall, the agreement between the experimental thermochemistry determined here and that calculated theoretically is found to be reasonable.

’ EXPERIMENTAL AND COMPUTATIONAL SECTION General Experimental Procedures. Cross sections for CID of the protonated molecules are measured using a GIBMS that has been described in detail previously.21,22 Protonated glycine is easily formed using an electrospray ionization (ESI) source23 under conditions similar to those described previously.23
26 Briefly, the ESI is operated using either H2O or 50:50 by volume H2O/MeOH solutions with ∼10
4 M glycine (all chemicals purchased from Sigma-Aldrich), syringe-pumped at a rate of 0.04 mL/h into a 35 gauge stainless steel needle biased at ∼2000 V. Ionization occurs over the ∼5 mm distance from the tip of the needle to the entrance of the capillary, biased at ∼35 V. Ions are directed by a capillary heated to 80
C into a radio frequency (rf) ion funnel,27 wherein they are focused into a tight beam. Ions exit the ion funnel and enter an rf hexapole ion guide that traps them radially. Here the ions undergo multiple collisions (>104) with the ambient gas and become thermalized. Ions produced in this source are assumed to have their internal energies well described by a Maxwell
Boltzmann distribution of rovibrational states at 300 K, as characterized in previous experiments.23,25,26,28 To generate the protonated dimer of glycine, a dc discharge flow tube (DC/FT) source is utilized.23,29
35 Briefly, ions are generated by a continuous dc discharge with typical operating conditions of 1.4
2.0 kV and 15
25 mA in a flow of ca. 10% argon in helium at a flow rate of 5000
7000 standard cubic centimeters per minute (sccm), with normal operating pressures of 0.3
0.4 Torr. About 50 cm downstream from the discharge, glycine is introduced into the flow tube using a temperaturecontrolled probe, which is heated to about 190
C. The complex ions of interest are formed via charge transfer/Penning ionization of glycine, followed by self-protonation and three-body association reactions in the flow of the He/Ar carrier gas. The complex ions are thermalized to 300 K (the temperature of the flow tube) both vibrationally and rotationally by undergoing ∼105 collisions with the buffer gases as they drift along the 1 m long flow tube.23,29
35 Ions are extracted from either source, mass selected using a magnetic momentum analyzer, decelerated to a well-defined kinetic energy, and focused into an rf octopole ion guide that

ARTICLE

traps the ions radially.36,37 The ion guide minimizes losses of the reactant and any product ions resulting from scattering. The octopole passes through a static gas cell containing xenon, which is used as the collision gas for reasons described elsewhere.38,39 After collision, the reactant and product ions drift to the end of the octopole where they are extracted and focused into a quadrupole mass filter for mass analysis. Ions are detected with a high voltage dynode and scintillation ion detector,40 and the signal is processed using standard pulse counting techniques. Ion intensities, measured as a function of collision energy, are converted to absolute cross sections as described previously.21 The uncertainty in relative cross sections is about (5%, and that for the absolute cross sections is about (20%. The ion kinetic energy distribution is measured to be Gaussian and has a typical fwhm of 0.1
0.3 eV (lab). Uncertainties in the absolute energy scale are about (0.05 eV (lab). Ion kinetic energies in the laboratory frame are converted to energies in the center-of-mass (CM) frame using ECM = Elabm/(m þ M), where M and m are the masses of the ionic and neutral reactants, respectively. All energies herein are reported in the CM frame unless otherwise noted. Thermochemical Analysis. Threshold regions of the CID reaction cross sections are modeled using eq 1 σ j ðEÞ ¼ ðnσ0, j =EÞ

X Z gi

E

E0, j
Ei

½kj ðEÞ=ktot ðEÞðE
εÞn
1 PD1 dðεÞ

ð1Þ where σ0,j is an energy-independent scaling factor for channel j; n is an adjustable parameter that describes the efficiency of collisional energy transfer;22 E is the relative kinetic energy of the reactants; E0,j is the threshold for CID of the ground electronic and rovibrational state of the reactant ion at 0 K for channel j; ε is the energy transferred from translation during the collision; and E* is the internal energy of the energized molecule (EM) after the collision, i.e., E* = ε þ Ei. The term kj(E*) is the unimolecular rate constant for dissociation of the EM to channel j, and its summation over all channels yields ktot(E*). PD1 is the probability for dissociation of the EM and is given by 1
exp[
ktot(E*)τ], where τ is the experimental time for dissociation (∼5  10
4 s in the extended dual octopole configuration as measured by time-of-flight studies).22 The summation in eq 1 is over the rovibrational states of the reactant ions, i, where Ei is the excitation energy of each state and gi is the fractional population of those states (∑gi = 1). This equation accounts for the lifetime for dissociation of the EM, which can lead to a delayed onset for the reaction threshold, a kinetic shift, which becomes more noticeable as the size of the complex increases. In addition, eq 1 naturally includes competition among parallel reactions in a full statistical treatment.41 Previous studies have verified its efficacy in modeling reactions that compete through loose as well as loose vs tight transition states.22,42
46 The rate constants kj(E*) necessary for competitive modeling41 and ktot(E*) are defined by Rice
Ramsperger
Kassel
Marcus (RRKM) theory as in eq 247,48 X X ktot ðEÞ ¼ kj ðEÞ ¼ dj Nj† ðE
E0, j Þ=hFðEÞ ð2Þ j

j

where dj is the reaction degeneracy of channel j; N†j(E*
E0,j) is the sum of rovibrational states of the transition state (TS) for channel j at an energy E*
E0,j; and F(E*) is the density of states of the EM at the available energy, E*. Vibrational frequencies and rotational constants are taken from quantum chemical calculations, as detailed 11145

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

in the next section. The Beyer
Swinehart
Stein
Rabinovitch algorithm49
51 is used to evaluate the number and density of the rovibrational states, and the relative populations gi are calculated for a Maxwell
Boltzmann distribution at 300 K. For reactions limited by loose TSs, most frequencies are just those of the products with the transitional frequencies treated as rotors, an approach that corresponds to a phase space limit (PSL), as described in detail elsewhere.41,52 The two-dimensional (2D) external rotations are treated adiabatically but with centrifugal effects included51 and calculated using a statistical distribution with an explicit summation over all the possible values of the rotational quantum number. For reactions limited by tight transition states, molecular parameters are taken from theoretical results. Because the decompositions of HþG and HþG2 also involve sequential dissociations, accurate modeling of the data requires additional assumptions to describe such processes, as described previously.53 The difficulty in analyzing sequential thresholds lies in the fact that the initial dissociation process takes away an unknown distribution of energies in translational modes of the initial products, as well as internal modes of the neutral product. This leaves an unknown distribution of internal energies in the ionic product that undergoes further dissociation. The procedure used to handle this effect uses eq 1 to reproduce the cross section for the product of the primary reaction, which excludes subsequent dissociation, σCID(E), combined with the probability for further dissociation, PD2 = 1
exp[
k2(E2*)τ2]. Here k2, E2*, and τ2 are the rate constant for the secondary dissociation, the energy available to the secondary EM, and the time available for the secondary dissociation, respectively. This partitions the total CID cross section into that for the nondissociating products, eq 3a, and that for the sequential dissociation product ion, eq 3b σ1 ðEÞ ¼ σ CID ðEÞð1
PD2 Þ

ð3aÞ

σ2 ðEÞ ¼ σ CID ðEÞPD2

ð3bÞ

where the rate constants are again calculated using RRKM theory, eq 2, for the new EM. The energy available to this EM is defined statistically, accomplished by methods described in detail elsewhere.53 The combination of sequential and competitive modeling allows accurate reproduction of all experimental reaction cross sections observed here, as detailed further below. Several effects that would otherwise obscure the interpretation of the data must also be accounted for during data analysis. The first effect involves energy broadening resulting from the thermal motion of the neutral collision gas and the kinetic energy distribution of the reactant ion, accounted for by explicitly convoluting the models of eqs 1 and 3 over both kinetic energy distributions.21 The second effect considers that our models only represent products formed as the result of a single collision event, accounted for by evaluating the cross sections for pressure effects and extrapolating to zero pressure of Xe (rigorously single-collision conditions) when necessary.54 After convolution with the kinetic energy distribution of the reactants, the model cross sections of eqs 1 and 3 are compared to the data. A nonlinear least-squares analysis is used to provide optimized values for σ0,j, n, and E0,j. The uncertainty associated with E0,j is estimated from the range of threshold values determined from different data sets with variations in the parameter n, 10% variations in the vibrational frequencies, changes in τ by factors of 2, and the uncertainty of the absolute energy scale, 0.05 eV (lab). For loose TSs, we assume that the

measured threshold E0 values for dissociation are from ground state reactant to ground state ion and neutral ligand products. Given the relatively long experimental time frame (∼5  10
4 s), dissociating complexes should be able to rearrange to their ground state product conformations upon dissociation. Computational Details. Model structures, vibrational frequencies, and energetics for all reaction species, including all transition state and intermediate species, were calculated using Gaussian 03.55 Series of relaxed potential energy surface (PES) scans at the B3LYP/6-31G(d) level were performed to identify the elementary steps. Transition state and intermediate structures occurring along the PESs were further optimized at the B3LYP/6-311þG(d,p) level, where each transition state was found to contain one imaginary frequency and each intermediate was vibrationally stable. Rotational constants were obtained from the optimized structures, and all vibrational frequencies were also calculated at this level. When used in internal energy determinations or for RRKM calculations, the vibrational frequencies were scaled by 0.99.56 Zero-point vibrational energy (ZPE) corrections were additionally determined using the scaled vibrational frequencies. Single-point energies were determined at the B3LYP, B3P86, and MP2(full) levels using the 6-311þG(2d,2p) basis set. To explore whether this standard approach is accurate, we performed additional calculations on G, HþG, CH2NH, CH2NH2þ, H2O, and CO such that results for the proton affinities (PA) of glycine and CH2NH and the reaction, HþG f CH2NH2þ þ H2O þ CO (reaction 5 below), could be compared with literature experimental results. In addition to the calculations noted above, geometry optimizations and vibrational frequency calculations were also performed on these species at the B3LYP/6-311þþG(2d,2p) and MP2(full)/6311þG(d,p) levels with the latter followed by single-point calculations at the MP2(full)/6-311þG(2d,2p). Results for these various approaches are compared in Table 1. In the following discussion, values at 0 and 298 K are converted using information listed in Table S1 of the Supporting Information, most of which is derived using the rigid rotor/harmonic oscillator approximation with rotational constants and vibrational frequencies calculated at the B3LYP/6-311þG(d,p) level. The uncertainties listed are determined by scaling the vibrational frequencies by (10%. Comparison of the various theoretical results for the two proton affinities shows little variation (Table 1). For PA(G), most 0 K values lie between 879 and 882 kJ/mol with the B3P86 result at 889 kJ/mol. The former values agree well with the recommended experimental value of Hunter and Lias,57 880.5 ( 8 kJ/mol at 0 K (886.5 kJ/mol at 298 K). For PA(CH2NH), the theoretical 0 K values lie between 858 and 863 kJ/mol with B3P86 at 868 kJ/mol. The Hunter and Lias compilation57 recommends 846.9 ( 8 kJ/mol (852.9 kJ/mol at 298 K), which agrees with an ICR measurement of 854 ( 8 kJ/mol (848 ( 8 kJ/mol at 0 K).58 A thermokinetic determination places the 298 K value at 862.9 ( 6.3 kJ/mol (856.9 ( 6.3 kJ/mol at 0 K),59 in reasonable agreement with theory. The discrepancy in the values for PA(CH2NH) can be evaluated further by more recent data. At present, the best value for the heat of formation of CH2NH2þ comes from a threshold photoelectron photoion coincidence spectroscopy (TPEPICO) study, 750.4 ( 1.3 kJ/mol at 298 K (762.4 ( 1.3 kJ/mol at 0 K).60 These values agree very well with a preceding high-level W2 calculation predicting 750.6 (762.6) ( 2.1 kJ/mol.61 This theoretical study also found the 298 (0) K heats of formation for CH2NH to be 88.2 (96.2) ( 2.1 kJ/mol. These values agree with 11146

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

Table 1. Comparison of 0 K Theoretical and Experimental Values for Proton Affinities of Glycine and CH2NH and the Heat of Reaction 5 this work a

process PA(G)

b

c

d

literature e

f

g

B3LYP

MP2

B3LYP//B3LYP

B3LYPþ

B3P86//B3LYP

MP2//B3LYP

MP2//MP2

878.8

881.3

881.4

881.0

888.8

879.6

878.9

880.5 ( 8h

858.4

846.9 ( 8 848 ( 8j

experiment

theory 925 (MP2)i 895 (G2)i

PA(CH2NH)

863.3

860.9

862.9

862.6

868.0

858.7

i

862.2 ( 3.0l

856.9 ( 6.3k þ

H G f CH2NH2

þ

137.1

130.7

137.1

136.9

184.1

136.6

134.7

þ H2O þ CO

139.0 ( 8.2m

199.2 (B3LYP)n 118.4 (QCISD)n 172.8 (MP2)n 154 (MP2)i

o

MAD

1.6

3.5

1.2

1.1

19.7

2.3

3.3

MADp

3.4

4.4

3.0

2.8

21.4

1.8

2.5

a

B3LYP/6-311þG(d,p). b MP2(full)/6-311þG(d,p). c B3LYP/6-311þG(2d,2p)//B3LYP/6-311þG(d,p). d B3LYP/6-311þþG(2d,2p). e B3P86/6311þG(2d,2p)//B3LYP/6-311þG(d,p). f MP2(full)/6-311þG(2d,2p)//B3LYP/6-311þG(d,p). g MP2(full)/6-311þG(2d,2p)// MP2(full)/6311þG(d,p). h Hunter and Lias, ref 57. i Uggerud, ref 8. MP2/6-31G(d,p) and G2(MP2). j Peerboom et al., ref 58. k Bouchoux and Salpin, ref 59. l W2 theory, ref 61. m See text. n O’Hair et al., ref 5. 298 K values adjusted to 0 K using information from Supporting Information, Table S1. B3LYP/631G(d), QCISD(T)/6-31þG(d,p)//B3LYP/6-31G(d), MP2/6-31G(d). o Mean absolute deviation from experimental values and W2 value for PA(CH2NH). p Mean absolute deviation from experimental values. PA(CH2NH) from ref 59.

298 K experimental values of 88 ( 17 kJ/mol obtained by photoionization mass spectrometry of pyrolysis products of azetidine.62 Holmes et al. used a similar approach with energyresolved electron ionization to obtain an upper limit of 92 ( 13 kJ/mol and, after evaluating the literature, recommended the same value, 88 ( 17 kJ/mol.63 Values as high as 110 ( 13 kJ/ mol64 and as low as 69 ( 8 kJ/mol58 have also been determined, as reviewed in the theoretical paper.61 None of the experimental values for the neutral heat of formation can be considered as definitive, so we adopt the W2 theory result. The W2 theoretical study also finds PA(CH2NH) = 868 (862) ( 3 kJ/mol at 298 (0) K, which agrees well with the calculations performed here, Table 1 (perhaps unsurprisingly). This calculation points to the higher experimental PA value as being more accurate. To calculate an experimental heat of reaction 5, the heats of formation for HþG and CH2NH2þ along with ΔfH298 (H2O) =
241.826 ( 0.042 kJ/mol and ΔfH298 (CO) =
110.53 ( 0.17 kJ/mol are needed.65 ΔfH298 (CH2NH2þ) is taken from the PEPICO study, and ΔfH298 (HþG) can be calculated by combining the experimental PA(G) with ΔfH298 (G). The latter has recently been reevaluated by Dorofeeva and Ryzhova and assigned as
393.7 ( 1.5 kJ/mol at 298 K,66 differing only slightly from previous values of
390.5 and
392.1 ( 0.6 kJ/mol.67,68 Combining this value with PA(G) = 886.5 ( 8 kJ/mol and ΔfH298 (Hþ) = 1530.05 ( 0.04 kJ/mol (ion convention for the enthalpy of the electron),65 the heat of formation of HþG is 249.85 ( 8.1 kJ/mol. Thus, the experimental heat of reaction 5 at 298 K can be calculated as 148.2 ( 8.2 kJ/mol, which can be adjusted to a 0 K value of 139.0 ( 8.2 kJ/mol using calculated vibrational frequencies and rotational constants for HþG and CH2NH2þ (Table S1, Supporting Information). This value is compared with calculations performed here in Table 1. The calculated 0 K values lie between 131 and 137 kJ/mol, with the B3P86 result at 184 kJ/mol. Overall, the agreement between theory and experiment is within the experimental uncertainties of about 8 kJ/mol for all approaches but B3P86/6-311þG(2d,2p)//B3LYP/6-311þG(d, p), which systematically overestimates these three experimental

values, being in particularly poor agreement with the heat of reaction 5. As a consequence, B3P86 values will not be considered below. B3LYP/6-311þþG(2d,2p) and B3LYP/6-311þG(2d,2p)// B3LYP/6-311þG(d,p) approaches give virtually identical results, indicating that use of this larger basis set for geometry optimizations is not necessary. Likewise, geometry optimizations at the MP2(full)/6-311þG(d,p) level give results very similar to those at the B3LYP/6-311þG(d,p) level, with the latter agreeing slightly better with experiment. If PA(CH2NH) is taken from Bouchoux and Salpin,59 the comparisons show that MP2 approaches are slightly better than B3LYP, whereas if the W2 value for PA(CH2NH) is used for comparison,61 the opposite result is obtained, but differences are only about 1 kJ/mol. Table 1 also contains results for reaction 5 taken from previous calculations regarding the decomposition of HþG by O’Hair et al.5 and Uggerud.8 O’Hair et al. performed calculations at the B3LYP/6-31G(d) and MP2/6-31G(d) levels as well as at the QCISD(T)/6-31þG(d,p) level using the B3LYP geometries and vibrational frequencies. The values listed for O’Hair et al. have been adjusted to 0 K values from the published 298 K results using Table S1 (Supporting Information). Uggerud used the MP2/6-31G(d,p) level of theory and also calculated PA(G) at this level and using the G2(MP2) approach. It can be seen that none of these approaches yields particularly accurate results, although the QCISD(T)5 and MP2/6-31G(d,p)8 values for reaction 5 are the best of these results. Nomenclature. As a means of identifying the various conformations of the HþG molecule, we use a nomenclature that specifies the site of protonation in brackets with O1 indicating the carbonyl oxygen and O2 the hydroxyl oxygen. This is followed by a designation of the two dihedral angles going from the N terminus to the hydroxyl group (i.e., — NCCO2 and — CCO2H), where c (cis) stands for angles 135
. Thus, the ground state conformer is HþG[N]-tt (see below). If the carbonyl oxygen is protonated, a third dihedral angle, — CCO1H, is also given. Transition states for proton transfer steps are named 11147

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

like TS[N
O1]-tt and those for dihedral angle rotations as TS[N]-t(tg). Although a bit more complicated than simply numbering the various species, we believe this nomenclature allows better visualization of the species and can be systematically extended to longer chains as well. For the HþG2 dimer, the conformation of each glycine monomer is independently indicated using the same method as for HþG.

’ RESULTS Cross Sections for Collision-Induced Dissociation. Kinetic energy dependent experimental cross sections were obtained for the interaction of Xe with HþG and HþG2. Figures 1 and 2 show representative data sets. Data shown are a mean of results taken at xenon pressures of ∼0.05, 0.1, and 0.2 mTorr, as no pressure dependences were detected within our experimental uncertainties. For the HþG system, the two ionic products observed, 48 and 30 m/z, correspond to CO loss and (H2O þ CO) loss to form the a1 ion, reactions 4 and 5, respectively.

Hþ G þ Xe f CH2 NH2 þ ðH2 OÞ þ CO þ Xe

ð4Þ

þ

f CH2 NH2 ða1 Þ þ H2 O þ CO þ Xe ð5Þ Loss of only CO begins at slightly lower energies, but subsequent loss of H2O is clearly efficient, which explains why the cross section for reaction 4 is small. This qualitative behavior is consistent with the ionic structures shown, the a1 immonium ion and its hydrated complex. We explicitly looked for the loss of H2O to form C2H4NOþ (58 m/z) but found no signal at this mass within the noise limit, corresponding to a cross section of 6 eV, CM), KK also observed the distonic ylide ion (þH3N
CH2•). Notably, Wesdemiotis and co-workers cited preliminary results of KK to determine that CH2NH2þ “is the fragment of least critical energy...showing that at threshold m/z 30 is not formed from m/z 48 via...sequential dissociation”.7 This conclusion is clearly the opposite of that deduced from the present work but is primarily a result associated with the enhanced sensitivity of the present experiments compared to those of KK, rather than a fundamental difference in the cross sections. For the HþG2 system, three ionic products are observed (Figure 2), consistent with reaction 6 followed by reactions 4 and 5. Hþ G2 þ Xe f Hþ G þ G þ Xe

ð6Þ

Figure 1. Cross sections for collision-induced dissociation of HþG with Xe as a function of kinetic energy in the center-of-mass frame (lower x-axis) and the laboratory frame (upper x-axis). Solid lines show the best fit to the data using the sequential model of eqs 1 and 3 convoluted over the neutral and ion kinetic and internal energy distributions. Dashed lines show the model cross sections in the absence of experimental kinetic energy broadening for reactions with an internal energy of 0 K.

Figure 2. Cross sections for collision-induced dissociation of HþG2 with Xe as a function of kinetic energy in the center-of-mass frame (lower x-axis) and the laboratory frame (upper x-axis). The solid line shows the best fit to the data using the model of eq 1 convoluted over the neutral and ion kinetic and internal energy distributions. The dashed line shows the model cross section in the absence of experimental kinetic energy broadening for reactions with an internal energy of 0 K.

Cross sections for CH2NH2þ(H2O) and CH2NH2þ show similar relative behavior as that in Figure 1 but are shifted up in energy, consistent with sequential processes. It perhaps is notable that we do not observe dehydration of the dimer to form HþGG, i.e., peptide bond formation to yield protonated glycylglycine. Such coupling reactions have been reported under chemical ionization conditions for methionine (Met) and glutamic acid (Glu).69 Theoretical Results for the Decomposition of HþG. The ground state (GS) structure of HþG has been well-characterized6,15,70
73 and confirmed by IRMPD measurements.15 11148

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

Protonation most favorably occurs at the amino nitrogen, with a hydrogen bond to the carbonyl group, such that the GS is designated as HþG[N]-tt in our nomenclature. If a hydrogen bond is formed with the hydroxyl group instead, the HþG[N]-ct structure is formed, which lies 19
22 kJ/mol higher in energy (Table 2). The HþG[N]tc structure retains the NH 3 3 3 OC hydrogen bond of the GS but rotates the hydroxyl group by 180
such that it no longer has an intramolecular interaction with the carbonyl, costing 30
35 kJ/mol. Protonation of the carbonyl oxygen (O1) leads to five distinct structures. HþG[O1]-tcc lies 60
68 kJ/mol above the GS and gains stability from a O1H 3 3 3 N hydrogen bond. Two HþG[O1]-tct structures that differ in the orientation of the NH2 group (anticlinal and synclinal) lie 96
109 and 98
111 kJ/mol above the GS, respectively, while a similar structure having the carboxylic acid group rotated by about 90
, HþG[O1]-gct, lies 5
8 kJ/mol higher still. If the two OH bonds point in the same direction, thereby interacting repulsively, HþG[O1]-ttt is formed and lies another 10
13 kJ/mol higher in energy. Protonation of the hydroxyl group (O2) gives two structures, HþG[O2]-tc and HþG[O2]-gg, lying 124
132 and 126
138 kJ/mol above the ground state, respectively. Both structures have long C
OH2 bond lengths (2.28 and 2.59 Å), such that they are more aptly characterized as an acylium ion stabilized by a water molecule. O’Hair et al.5 identified all of these conformers except HþG[O1]-tct(HNsyn), obtaining similar relative energetics at the QCISD(T) level (Table 2). Uggerud characterized only a couple of these species8 (Table 2). Early computational work concluded that the aminoacetyl ion formed by loss of water from HþG has a CH2NH2þ(CO) structure.74,75 More complete explorations of the elementary steps for (H2O þ CO) loss from HþG have been conducted by KK, Uggerud, and Balta et al.,6,8,9 with the most comprehensive

theoretical study conducted by O’Hair et al. at the B3LYP/631G(d) and MP2(fc)/6-31G(d) levels of theory.5 O’Hair et al. found that the lowest-energy pathway involves transfer of the proton from the amino group to the hydroxyl group, which rearranges to form a CH2NH2þ(H2O)(CO) complex that can decompose to yield the observed products. The pathway is similar to that identified by Uggerud but more correctly connects all transition states and intermediates (as verified here). Using the higher level theoretical methods outlined above, we have reexamined the potential energy surface (PES) of elementary steps located by O’Hair et al. for this reaction. The 0 K PES is shown in Figure 3 with relative energies of all intermediates and transition states listed in Table 2. Starting with the HþG[N]-tt ground state, the first transition state, TS[N]-(tc)t, involves rotation about the CC bond leading to formation of the HþG[N]-ct intermediate. In TS[N
O2]-ct, the proton transfers from the amino nitrogen to the OH group forming HþG[O2]-gg, which can also be characterized as (NH2CH2COþ)(H2O), a complex of an acylium ion stabilized by the water ligand. The barrier to cleave the C
CO bond (indicated by the ∼ symbol) over TS(NH2CH2þ∼CO)(H2O) is very small, only 0.5
3.9 kJ/mol including zero-point energy corrections (Table 2). Note that TS[N
O2]-ct is rate limiting, lying 2
5 kJ/mol above TS(NH2CH2þ∼CO)(H2O). Intrinsic reaction coordinate (IRC) calculations indicate that TS(NH2CH2þ∼CO)(H2O) forms a bis-ligand complex of the final products, (COHC)CH2NH2þ(H2OHN), in which the CO is hydrogen bound to the HC bond of CH2NH2þ and the water is bound to an HN bond (as indicated by the subscripts). A species having a very similar energy in which the CO is bound to the carbon atom of CH2NH2þ was also located, (COC)CH2NH2þ(H2OHN), and is

Table 2. Theoretical Energies (kJ/mol) at 0 K Including Zero-Point Corrections for Intermediates and Transition States on the HþG Surface Relative to the Ground State this work species

a

B3LYP

literature

b

c

B3LYP//B3LYP

MP2(full)//B3LYP

O’Hair et al.

d,e

Uggerude, f

HþG[N]-tt

0.0

0.0

0.0

0.0 (1)

HþG[N]-ct

18.7

22.0

18.8

15.9 (1a)

HþG[N]-tc HþG[O1]-tcc

34.5 62.3

30.6 59.7

31.8 68.2

36.8 (5a) 70.7 (6a)

HþG[O1]-tct(HNanti)

100.0

95.7

108.5

111.3 (4a)

HþG[O1]-tct(HNsyn)

102.1

98.4

110.6

HþG[O1]-gct

107.0

103.1

118.3

119.4 (3a)

HþG[O1]-ttt

120.3

114.8

128.7

131.4 (7a)

HþG[O2]-tc

126.5

131.8

124.0

143.9 (4b)

HþG[O2]-gg

133.3

138.1

125.8

146.0 (1b)

TS[N]-(tc)t TS[N
O2]-ct

33.3 138.5

34.8 144.1

32.6 131.5

27.6 (1ats) 149.0 (1bts)

147 (2
4)

TS(NH2CH2þ∼CO)(H2O)

133.8

139.0

129.7

138.9 (1cts)

147 (4
5)

0 (2)

124 (3)

(COHC)CH2NH2þ(H2OHN)

49.0

54.6

46.1

39.7 (3c)

(COC)CH2NH2þ(H2OHN)

48.9

54.6

45.3

39.3 (1c)

(COHN)CH2NH2þ(H2OHN)

39.2

44.5

34.4

141 (4)

57 (5)

CH2NH2þ(H2OHN) þ CO

58.0

63.9

60.2

47.3 (1d)

CH2NH2þ(COHN) þ H2O

112.3

111.8

105.1

104.2 (1e)

144 (8 þ 10)

CH2NH2þ þ H2O þ CO

137.1

137.1

136.6

127.6 (1f)

163 (7 þ 8 þ 9)

71 (6 þ 9)

a

B3LYP/6-311þG(d,p). b B3LYP/6-311þG(2d,2p)//B3LYP/6-311þG(d,p). c MP2(full)/ 6-311þG(2d,2p)//B3LYP/6-311þG(d,p). d Ref 5. 298 K values. QCISD(T)/6-31þG(d,p)//B3LYP/6-31G(d). e Numbers in parentheses identify the species name used in this reference. f Ref 8. MP2/631G(d,p). 11149

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

Figure 3. Reaction coordinate surface for (H2O þ CO) loss from HþG. Geometry optimizations and single-point energies of each elementary step are determined at the B3LYP/6-311þG(d,p) level of theory and corrected for ZPE. Short dashed lines indicate hydrogen bonds. Long dashed lines indicate bonds that are breaking.

the species identified by O’Hair et al. as the initial intermediate formed. A more stable variant of this complex, (COHN)CH2NH2þ(H2OHN), in which the CO binds to the HN bond was also optimized in the present study and lies 10
14 kJ/mol lower in energy. Several transition states for moving the CO around the CH2NH2þ(H2O) complex were located (not included in Table 2) and lie only about 1 kJ/mol above (COHC)CH2NH2þ(H2OHN) such that this complex can collapse to (COHN)CH2NH2þ(H2OHN) easily. Any of these intermediate complexes can dissociate by CO or H2O loss. Figure 3 and Table 2 show that CO loss from the complex is more favorable than H2O loss by 45
54 kJ/mol, consistent with the observation of CO loss (48 m/z) but not H2O loss (58 m/z), both here and in previous experimental studies.6,7,70 According to the calculations, the loss of CO to form CH2NH2þ(H2O) (48 m/z) is limited by TS[N
O2]-ct, which has an energy very close to that for the overall product asymptote forming CH2NH2þ (30 m/z) þ H2O þ CO. At the B3LYP level of theory, TS[N
O2]-ct lies above this asymptote by 1
7 kJ/mol, whereas MP2(full) calculations indicate the opposite: the product asymptote lies above TS[N
O2]-ct by 5 kJ/mol. In either case, it is clear that formation of the 48 and 30 m/z products is anticipated to have similar threshold energies, in agreement with our observations. We also examined what happens if water is lost from the acylium ion intermediate, HþG[O2]-gg or (NH2CH2COþ)(H2O). A relaxed potential energy surface scan for loss of H2O from this intermediate shows that as the water moves away the energy rises slightly (a couple of kilojoules/mole), but then the remaining acylium ion spontaneously decomposes by CO loss. No transition state could be localized despite repeated attempts to find one. This is in agreement with early work suggesting that the NH2CH2COþ acylium ion is unstable with respect to the loss of CO.1,76,77 The theoretical energies obtained for TS[N
O2]-ct and the CH2NH2þ þ H2O þ CO products can be compared to results for previous calculations conducted at lower levels of theory by

Uggerud (MP2(FC)/6-31G(d,p) including zero-point corrections) and O’Hair et al. (B3LYP/6-31G(d), QCISD(T)/631G(d,p)//B3LYP/6-31G(d), and MP2(FC)/6-31G(d)). As noted above, the lower level B3LYP and MP2 calculations do not yield very accurate results for our calibration reactions (Table 1) and generally yield much higher energies than those provided in Table 2. Therefore, only the QCISD(T) results of O’Hair et al. and the MP2 results from Uggerud are included in Table 2. These results can be seen to be comparable to the present calculations, with the most egregious difference being for the CH2NH2þ(CO) þ H2O products calculated by Uggerud. This is because these calculations refer to a CH2NH2þ(COC) complex, rather than the much lower energy CH2NH2þ(COHN) complex located here and by O’Hair et al. Overall, all of these calculations provide the same qualitative potential energy surface. Analysis of Cross Sections for HþG Decomposition. The potential energy surface of Figure 3 shows that the primary 48 m/z fragment ion sequentially loses H2O to form the 30 m/z ionic product. Therefore, sequential modeling using eqs 1 and 3 can provide the thresholds for both reactions. In this case, formation of the CH2NH2þ(H2O) (48 m/z) product is assumed to be limited by TS[N
O2]-ct. This product is then assumed to go on to dissociate to CH2NH2þ (30 m/z) þ H2O over a loose transition state corresponding to the separated products. It should be realized that the statistical sequential model,53 which was developed primarily for sequential loss of ligands over loose TSs, is not completely appropriate in this case, in particular because of the large drop in energy from TS[N
O2]-ct to the (CO)CH2NH2þ(H2O) complexes that undergo further decomposition. Nevertheless, Figure 1 shows that the sequential model reproduces the experimental cross sections of both products over a large range of energies (>2 eV) and magnitudes (over 2 orders of magnitude). The optimized fitting parameters of eqs 1 and 3 are provided in Table 3. The threshold for loss of CO lies at 160 ( 5 kJ/mol and should correspond to the energy of TS[N
O2]-ct. Consistent with the appearance of the cross 11150

dx.doi.org/10.1021/jp2025939 |J. Phys. Chem. A 2011, 115, 11144–11155

The Journal of Physical Chemistry A

ARTICLE

Table 3. Fitting Parameters of Equation 1,a reactant HþG

reaction

σ0

TS

H G2

E0 (eV)

CH2NH2þ(H2O) þ CO

TS[N
O2]-ct

5.2 (0.7)

1.5 (0.2)

1.66 (0.05)

CH2NH2þ þ H2O þ CO

looseb

5.4 (0.8)

1.5 (0.2)

1.82 (0.05)

0.9 (0.2)

1.45 (0.08)

loosec þ

n

þ

H GþG

loose

8.7 (0.5)

1.0 (0.1)

1.21 (0.08)

CH2NH2þ(H2O) þ CO þ G

TS[N
O2]-ct

102 (15)

0.6 (0.1)

2.77 (0.08)

a Uncertainties are in parentheses and are two standard deviations for E0. b Sequential model of the data using eqs 1 and 3. c Derived from fitting the branching ratio as described in the text.

section data, the threshold measured for loss of both H2O and CO,