J. Phys. Chem. 1995, 99, 2278-2291
2278
Franck- Condon Dominated Chemistry. Formation and Dissociations of Tetrahydroxyphosphoranyl Radicals following Femtosecond Reduction of Their Cations in the Gas Phase FrantiSek TureEek,*$'Ming Gu,l and Cornelis E. C. A. Hop2 Departments of Chemistry, BG-10, University of Washington, Seattle, Washington 981 95, and University of Wisconsin, Madison, Wisconsin 53706 Received: June 28, 1994; In Final Form: November 22, 1994@
The gas-phase chemistry of phosphorus oxoacids, radicals, cations, and cation radicals is investigated by experiment and theory. Vertical neutralization of the stable P(OH)4+ cation forms the P(OH)4*radical which dissociates exothermically within 4.5 ps by loss of hydrogen to form phosphoric acid. Collisional reionization of vibrationally excited H3P04 results in extensive dissociation by losses of hydroxyl groups. Vertical neutralization of the stable P(OH)3'+ and HPO(OH)2'+ ions forms stable molecules that are detected as survivor ions following reionization. Collisional activation of neutral trihydroxyphosphine, P(OH)3, results in unimolecular isomerization to the more stable phosphorous acid, HPO(OH)2. Ab initio calculations, carried out at the MP4(SDQ)/6-3 1+G(d)//MP2(FULL)/6-3 1+G(d) level of theory predict all the species under study to be stable equilibrium structures. The proton affinity of phosphoric acid is calculated as 807 kJ mol-'. Vertical neutralization of P(OH)4+ deposits 129 kJ mol-' in the radical formed making it kinetically unstable with respect to hydrogen loss that requires 90 kJ mol-'. The most stable P(OH)3 isomer (Cl)is 45 kJ mol-' less stable than HPO(OH)2 but is separated by an isomerization bamer of 224 kJ mol-'. The P(OH)3" cation radical of C3 symmetry is 136 kJ mol-' more stable than HPO(OH)2'+. Vertical neutralization of P(OH)3'+ forms a vibrationally excited neutral 103 kJ mol-' above the C1 isomer, while neutralization of HPO(OH)*'+ results in a 98 kJ mol-' excitation in the neutral phosphorous acid. The important role of Franck-Condon effects in the dissociations and isomerizations of gas-phase phosphorus oxoacids and radicals is discussed.
Phosphorus oxoacids, esters, and anions play an important role in chemistry and biochemistry, as studied extensively by experiment3 and t h e ~ r y . In ~ contrast to the chemistry of phosphorus oxoacids in solution that has been studied for decade^,^ very little is known about the properties of free phosphorus oxoacids in the gas phase. Related to phosphorus oxoacids, transient molecules and radicals of low-valent phosphorus oxides and hydroxides have been of considerable recent theoretical interest.6 Oxygenated phosphorus radicals have been considered as intermediates in phosphine oxidation,' and hydroxyl radical attack in alkyl phosphites,* and these radicals may possibly be formed by P-0 bond homolysis in phosphorus oxoacids and esters of biological imp~rtance.~ A number of tetravalent phosphorus radicals, mostly with sulfur or halogen substituents, have been generated in solution or solid matrix, and their properties have been examined by electron spin resonance spectroscopy.lo Hydroxyphosphoranyl radicals of the P(OH),&-, type have been studied extensively by ab initio calculations that revealed a large number of both stereoisomers differing in the relative positions of the semioccupied molecular orbital (SOMO) and the substituents, and rotamers differing in the torsional angles about the P-0 bonds6 Cramer and Gustafson systematically investigated the relative stabilities and energy barriers to interconversion of trigonal bipyramidal structures in tetracoordinated phosphoranyl radicals in which the unpaired electron is typically presumed to prefer the equatorial positiom6 We have recently reported' that bicoordinated phosphorus radicals, e.g., H3COP'OH and CH30POCH3, can be generated as stable species in the gas phase and analyzed by neutraliza-
'
Abstract published in Advance ACS Abstracts, February 1, 1995.
tion-reionization mass spectrometry (NRMS). l 2 In this technique a stable cation or anion is generated in the gas phase, accelerated to a kiloelectronvolt kinetic energy, selected by mass, and then allowed to collide with a thermal atom or molecule. Neutral products formed by collisional electron transfer are separated from the residual ions and allowed a few microseconds to dissociate. The surviving stable or metastable neutral species are reionized by another collision and detected as stable ions or their dissociation products to give the neutralizationreionization mass spectrum. Due to the short interaction time s) the electron transfer is in the neutralizing collision essentially a vertical process forming the nascent neutral species with a geometry that closely resembles that of the precursor ion. For an ion and the corresponding neutral molecule of substantially different equilibrium geometries in their ground electronic states, vertical neutralization results in vibrational excitation through Franck-Condon effects that may facilitate d i ~ s o c i a t i o n . ' ~Due ~ ~ ' ~to the nonresonant nature of electron transfer at kiloelectronvolt collision energies, excited states of the neutral intermediate can be accessed and contribute to dissociations.l 4 Intrinsic stabilities and Franck-Condon effects play an important role in collisional neutralization of tricoordinated phosphorus cations and cation radicals such that the intermediate radicals and molecules undergo extensive dissociations by P-0 and C - 0 bond cleavages." This behavior raised the question of the stability of tetracoordinated oxygenated phosphorus radicals that have been predicted by theory to exist as bound structures in potential energy minima6 but have not been prepared so far. We now report on the generation and unimolecular dissociations of the tetrahydroxyphosphoranyl radical, P(OH)4', 1, its deuterium labeled isotopomers P'(OD)4
0022-3654/95/2099-2278$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 8, 1995 2279
Tetrahydroxyphosphoranyl Radicals (la), P(OD)3(0H) (Ib), P.(OD)z(OHh (IC),and P(OD)(OH)3 (Id), its methyl ester P(OH)(OCH& (a), and its isotopomer P(OH)(OCD3)3 (6a). The prototypical radical 1 represents a 9-P-4 hypervalent species that can be viewed as a product of hydrogen atom attachment to phosphoric acid or an intermediate in hydroxyl radical attack on trihydroxyphosphine. We show that the formation of radical 1 in the gas phase is dominated by very large Franck-Condon effects in the reduction step. We also investigate by experiment and ab initio calculations the dissociation paths for P - 0 and 0 - H bond cleavages in 1 and identify their products. Experimental Part Materials. Triethyl phosphate, trimethyl phosphate, triethyl phosphite, anhydrous (99%) phosphoric acid, phosphorous acid, and dimethyl disulfide were purchased from Aldrich and used as received. Liquid samples were degassed through several freeze-thaw cycles before use. Xenon (Airco, 99.95%) and trimethylamine (Matheson, 99%) were used as received. Trimethyl-& phosphate and trieth~1-d~ phosphate were prepared from POCl3 and CD30D or C2D50D, respectively, according to a standard literature pr0~edure.l~The mixed esters (C2H50),(C2D50)3-,PO were prepared by the same standard procedure using a 1.5:1 molar mixture of C2D5OD and C2H5OH. Methods. Neutralization-reionization (+NR+) spectra were obtained on a tandem quadrupole acceleration-deceleration mass spectrometer described previously.l6 Volatile samples were introduced into the ion source from a small glass reservoir maintained at 25 "C; the sample intake was regulated by a Teflon needle valve to achieve pressures in the range of 8 x Torr. Phosphoric and phosphorous acids were to 2 x introduced from a heated glass probe reaching into the ionization chamber. Precursor ions were generated by electron impact ionization of triethyl phosphate at 70 or 15-25 eV as specified below, or by gas-phase protonation with C4H9+ of trimethyl phosphate in a tight ion source of our design. The chemical ionization conditions were as follows: emission current, 1 mA; electron energy, 100 eV; isobutane pressure, 2 x Torr as read on the ionization gauge located outside the ion source. The reagent gas pressure and ion source potentials were adjusted to optimize protonation and minimize electron ionization such as to obtain ion abundance ratios of [M H]+/[M'+] > 20. The precursor ions were accelerated to 8200 eV and neutralized by collisions with gaseous CH3SSCH3, (CH3)3N, NO, or Xe at pressures such as to achieve 70% transmittance of the precursor ion beam. The remaining ions were separated from the neutral products, and the latter were reionized by collisions with oxygen at 70% transmittance of the precursor ion beam. The intermediate neutral lifetimes were in the 4.5-6.1 ps range for precursor ions of d z 80-150. Collisionally activated dissociation of the intermediate neutrals (+NCR+) was carried out by admitting helium into the differentially pumped neutral drift region16b,c at a pressure such as to achieve 50% transmittance of the precursor ion beam. The drift region was floated at +250 V, so that any ions formed there had kiloelectronvolt total energies and were rejected by an energy filter.16a The spectra reported in the figures and the data in Table 1 were averaged over 2530 repetitive scans obtained at scan rates of 1 s (75 data points) per mass unit. Metastable-ion dissociations of mass-selected ions of 8 keV kinetic energy were measured in the second field-free region of a VG ZAB-2F double-focusing mass spectrometer having a reversed (magnet B precedes electrostatic sector E) geometry. The kinetic energy release values were measured under conditions of good energy resolution, and the metastable peak width
+
was corrected for the width of the precursor main beam ( e 3 eV). Collisionally activated dissociation (CAD) spectra were obtained in a collision cell located in the second field-free region of the VG ZAB-2F instrument. Helium was used as the collision gas at a pressure to achieve 75% transmittance of the precursor ion beam. In these measurements all slits were fully open to obtain maximum signal intensity and to minimize energy resolving effects. The metastable-ion and CAD spectra were obtained as single scans. CAD spectra of 4 keV ions were also measured on a Kratos Profile HV-4 instrument equipped with a collision cell of our design that was mounted in the first fieldfree region and grounded. Oxygen at 70% transmittance of the precursor ion beam was used as the collision gas. The spectra were obtained by scanning the magnet (B) and electrostatic (E) sectors while maintaining a constant B E ratio ( B E linked scan). Calculations. Standard ab initio calculations were carried out using the Gaussian 92 set of programs.17 Spin-unrestricted calculations (UHF)were used for open-shell species. In calculations of reaction paths with the open-shell species, standard SCF iterations17 sometimes failed to converge in the regions of high potential energy, but satisfactory results could be obtained with the quadratic convergence method.'* Geometries were fist optimized with the 6-31G(d) or 6-31+G(d) basis sets to obtain local minima characterized by harmonic vibrational analysis (all frequencies real) or first-order saddle points (one imaginary frequency). The geometries of stationary points were then reoptimized with the 6-31+G(d) basis set including perturbational M ~ l l e r - P l e ~ s etreatment t~~ of electron correlation effects truncated at second order (MP2(FULL)).20 Zero-point vibrational energies and 298 K enthalpies, the latter calculated within the rigid rotor-harmonic oscillator approximation, were obtained from the harmonic vibrational frequencies calculated with the 6-31G(d) and 6-31+G(d) basis sets and scaled by O.8gs2O The unscaled harmonic frequencies are given in Chart 1. Improved total energies were obtained by single-point calculations on the MP2(FLJLL)/6-31+G(d) optimized geometries using the Moller-Plesset theory (frozen core) truncated at fourth order with single, double, and quadruple excitations (MP4(SDQ)). MP3 total energies were checked and in all cases confirmed convergence of the MP series. Effects of limited basis set size were taken into account in MP2 (frozen core) single-point calculations carried out with the large 6-31 l+G(3df,2p) basis set.21 These single-point calculations were used for energy corrections in the H3P03 and HzPo3 systems assuming the linear relationship22given by eq 1. AE{MP4(SDQ)/6-3 1I+G(3df,2p)} = E{MP4(SDQ)/6-3 1+G(d)} [E{MP2/6-311+G(3df,2p)} - E{MP2/6-31+G(d)}]
+
(1)
The use of sufficiently large basis sets is critical for calculations of accurate ionization and recombination energies as we reported recently.16c
Results and Discussion
Ion Characterization. The P(OH)4+ precursor ions (1+)that were used for the generation of radical 1 are formed by dissociative ionization of a variety of alkyl phosphate^.^^ In this work we used the triethyl phosphate cation radical which eliminates successively a vinyl radical and two ethylene molecules to give P(OH)4+ as the most abundant fragment ion (Scheme l).24This fragmentation sequence was c o n f i i e d by metastable-ion dissociations of the molecular ion and the intermediate fragment ions at d z 155 and 127 that all showed the eliminations depicted in Scheme 1 as the predominant (-95%) dissociation paths. The isotope pattern at d z 99-
2280 J. Phys. Chem., Vol. 99, No. 8, 1995
TureEek et al.
CHART 1: Calculated Uncorrected Harmonic Vibrational Frequencies
P(0H)a' (1, C, UHF/6-31G(d)): 104, 234, 279,477,484,495, 513,554,585,669,844,900,954, 1141, 1153, 1207, 1267, 4026,4028,4096,4127 cm-'. TS((H0M'O-H' (CI, UHF/6-31+G(d)): 2339i, 183, 249, 299, 359,402,434,441,482,507,834,924, 1022, 1068,1096, 1126, 1164, 1284,3915,3972,4068 cm-'. P(OH)d+(I+,Dz,RHF/6-31G(d)): 268, 290,343,343, 393,445, 465,465,506,942, 1004, 1017, 1017, 1017, 1171, 1222, 1222, 3990,3991,3991,4008 cm-'. PO(OH)Z-+OH~(2+, CI, RHF/6-31G(d)): 209,242,303,346, 385,408,449,471,476,583,921,991, 1009, 1078, 1202, 1540, 1788,3864,3965,3984,3993 cm-'. H?P04 (3, C3, RHF/6-31G(d)): 169(E),305(A), 404(E), 495(A), 505(E), 91l(A), 1024(E), 1147(A), 1149(E), 1436(A), 4087(E), 4093(A) cm-'. H3POa" (3*+,CI, UHF/6-31+G(d)): 201,278, 309, 359, 399, 439,443,473,915,948,959,981,1114, 1212, 1233,3964, 3967,3980 cm-'. P(OH)3 (4, C1, RHF/6-31G(d)): 185,227, 330,423,471,504, 863,907,920, 1081, 1106, 1193,4050,4056,4112 cm-I. TS(HPO(OH)*-P(OH)j) (Cr, RHF/6-31+G(d)): 2187i. 109, 305,389,425,551,618,940,962, 1072, 1104, 1213,2321, 4089,4089 cm-l. P(OH)i'+ (4'+, C3, UHF/6-31G(d)): 224(E), 291(A), 393(E), 497(A), 942(E), 963(A), 995(A), 1174(E),3970(E), 3985(A) cm-'. HPO(OH)z'+ (5'+, Cs, UHF/6-31G(d)): 251,312,336,438,472, 889,898, 1005, 1094, 1120, 1140, 1206,2935, 3967, 3980 cm-l. P(O)(OH)2+( C ZRHF/6-31+G(d)): ~ 415(B2), 425(A1),454(B l), 505(A2), 592(B1), 935(B2), 1004(Al), 1055(Al), 1281(B2), 1591(A1),3897(B2), 3911(A1) cm-I. P(O)(OH)2' (CnUHF/6-31+G(d)): 146(A"), 210(A'), 387(A"), 417(A'), 524(A'), 913(A'), 953(A"), 1086(A'), 1087(A"), 1350(A'), 4069(A"), 4071(A') cm-'. P(OH)2' (Cz, UHF/6-31+G(d)): 134(B),298(A), 449(A), 865(B), 879(A), 1011(B), 1049(A), 4059(B), 4061(A) cm-'.
100
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SCHEME 1
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Figure 1. (a, top) CAD ( 0 2 , 70% transmittance) spectrum of 1+. +NRf (CH3SSCH3 (70% T)/Oz (70% T)) spectra of (b, middle) 1+ and (c, bottom) la+.
mz 99
iivz 127
101 in the 70-eV electron ionization mass spectrum of triethyl phosphate, namely, the lack of isotope satellites at d z 100,11325 shows no detectable isobaric contaminants at d z 99, as also confirmed by a high-resolution mass spectrum. Ion 1+ gives a simple collisionally activated dissociation (CAD) mass spectrum that shows a dominant ion fragment due to loss of water ( d z 81), ions due to losses of hydrogen atom ( d z 98) and hydroxyl radical ( d z 82), and secondary fragments at d z 63 and 47 (Figure la). These dissociations are compatible with the presumed24bond connectivity in P(OH)4+, with the elimination of water resulting from a hydrogen transfer rearrangement typical for dissociations of even-electron ions? However, the elimination of water could also be due to dissociations of other isomers differing in the position of hydrogen atoms, e.g., O=P(OH)20Hz+, or a proton bound complex, (H0h-O. OH+-OH, that in fact may represent intermediates for water elimination from isomer 1+. The metastable-ion spectrumz7 of 1+ shows a dominant peak for the loss of water of moderate kinetic energy
-
release, T(0.5) = 22 f 1 meV. The peak profile is fitted very well with a single Gaussian function, H(w)= Ho exp(-awfl) of n = 2.03,27awhich gives 1.2% standard deviation at 10-90% peak heights, indicating a single structure for the dissociating l+.27d The value for T(0.5) and the average kinetic energy release (Tav= 4.5 kJ are comparable to the kinetic energy release in eliminations of water from other even-electron ions, e.g., CH3CH2CH=OHf and (CH&CH=OH+ 27d but significantly greater than kinetic energy release in loss of water from ion-molecule The formation of P(OH)4+ from triethyl phosphate by sequential elimination of C2H3' and two ethylene molecules involves transfer of four hydrogen atoms (Scheme 1). Such a multistep rearrangement is likely to produce the most stable isomer by analogy with other ion dissociations involving multiple hydrogen transfer^.^^,^^ The relative stabilities of Pod+ isomers thus can provide supporting evidence for the structure produced from triethyl phosphate. While no experimental thermochemical data on H4P04+ ions are available,29 ab initio calculations (Table 2 ) show at least two stable isomers, e.g., 1+ and O=P(OH)20+H2 (2+) (Figure 5). However, 1+ is calculated to be 123 kJ mol-' thermodynamically more stable
a-
J. Phys. Chem., Vol. 99, No. 8,1995 2281
Tetrahydroxyphosphoranyl Radicals
TABLE 1: Neutralization-Reionization (+NR+, CH3SSCH3 (70% transmittance)/Oz (70% transmittance)) and Neutralization-Collisional Activation-Reionization (+NCR+,CH~SSCHB (70% transmittance)/He (50% transmittance)/Oz (70% transmittance)) Mass Spectral Data for 1 and la-lc relative intensityn 1 ( d z 99) l a ( d z 103) l b ( d z 102) IC ( d z 101) Id ( d z 100) mlz +NR+ +NCR+ +NR+ +NCR+ +NR+ +NCR+ +NR+ +NCR+ +NR+ +NCR+ 103 102 0.2 0.06 0.3 101 100 0.05 0.05 0.02 0.03 0.1 0.07 99 0.1 98 0.1 0.2 85 0.1 84 0.2 83 2 0.2 0.2 0.1 1.5 0.2 0.3 0.4 0.5 0.4 0.3 82 0.5 81 1.5 2.3 3 0.7 0.5 0.8 0.8 0.9 2.2 1 0.9 1.3 0.8 80 2.9 0.5 1.3 3 0.4 0.8 0.6 0.08 79 0.4 0.4 67 1 0.4 3.4 3.9 2 0.9 1.8 0.8 65 0.5 1.8 1.6 1.5 0.9 2.5 64 2.8 1.3 4.7 2.6 4.9 5.6 3.5 4.1 4.8 5.8 5 7.5 63 0.6 0.2 1 2.9 2.5 1.1 0.4 1.4 49 1.3 0.6 0.4 1.4 1 48 0.9 0.5 1.1 44 42 48 45 46 47 49 59 50 47 63 24 31 33 21 26 22 19 20 19 31 19 0.2 3.6 0.4 0.4 0.9 3 20 1.2 0.4 0.2 0.9 0.8 2.3 19 2.4 4 3.1 0.5 0.6 7.9 8.9 3.5 2.9 5.9 18 2.6 2.8 5.5 3.4 3.5 2.0 17 1 1.3 8.6 8.7 8.5 7.3 8.1 16 2.7 5.7 6.7 7.2 6.1 Relative to the sum of reionized ion intensities, % x+NR+. than 2+, so formation by hydrogen transfer rearrangements of 1+ should be preferred. Coformation with 1+ of a less stable isomer would likely depend on the triethyl phosphate ion internal energy, with 1+ being preferred at low excitation. However, the +NR+ mass spectra of 1+ obtained at 70 and 20 eV are identical within experimental error (see below), indicating structure homogeneity of the ion in line with its metastable-ion spectrum. Neutralization-Reionization Spectra. Collisional neutralization of stable 1+ with xenon, nitric oxide, trimethylamine, and dimethyl disulfide followed by reionization with oxygen results in extensive dissociation such that the survivor ion corresponding to reionized 1+ is absent in all +NR+ mass spectra. Neutralization with dimethyl disulfide showed the highest efficiency as judged by the signal to noise ratios in the +NR+ spectra, and so this reagent was used consistently as an electron donor in the generation of all neutral species under study. The CH3SSCH3/02 +NR+ spectrum of 1+ (Figure lb) is dominated by the PO fragment at d z 47 (63% of sum of +NR+ ion intensities, C+NR+, Table l), which is typical for dissociations of other oxygenated phosphorus radicals." The highest mass fragment at m/z 98 (0.14% C+NR+) corresponds to H3P04 formed from 1 by loss of hydrogen. Surprisingly, on neutralization 1 does not lose a hydroxyl group to give a detectable product at d z 82, in contrast to CAD of ion 1+ which does (Figure la). The H2P03+ ion at d z 81 can be accounted for by water elimination from neutral 1 or ion 1+ following reionization, the latter corresponding to CAD of the ion (Figure la), or by loss of hydroxyl radical from neutral H3P04 or its ion following reionization as summarized in Scheme 2. Out of these possible formations, loss of water from neutral 1 appears to be inefficient as indicated by the very low relative intensity of the peak of reionized H20'+ at d z 18 in the +NR+ mass spectrum (0.5% C+NR+, Figure lb). This phenomenon is further discussed below. The accompanying HP03'+ ion at d z 80 formally corresponds to loss of H3O from 1 and must be
SCHEME 2 CH3SSCH3
P(OH)4+
P(W4' 1
1+
4+'
therefore due to secondary neutral or ionic dissociations of H3PO4 andor H2P03. Collisional activation with helium of the neutralization products (+NCR+) results in relatively small changes in some reionized product relative intensities (Table 1). Albeit small, these changes are outside the experimental error in ion relative intesity measurements. For most fragments, neutral excitation induces hydrogen losses as judged from the increased [0'+]/ [OH+], [POfl/[POW+l, and [P02+]/[HP02'+] abundance ratios. Note that the peak at d z 32, which also slightly increases on +NCR+, is in part due to collisional reionization of thermal oxygen molecules in the electrostatic potential gradient of the deceleration lens.16a In contrast to the case of lower mass fragments, the [HP03*+]/[H2P03+]ratio slightly decreases on neutral excitation, possibly due to more efficient formation of H2PO3. However, this effect is likely to be counterbalanced by different extents of further dissociation of H2PO3. and HPo3, as discussed below. Phosphoric acid (3) is one of the primary products of 1 dissociation. While 3 undoubtedly is a stable molecule in the gas phase,4dthe stability and dissociations of its cation radical
TureEek et al.
2282 J. Phys. Chem., Vol. 99, No. 8, 1995 47
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Figure 2. 70-eV electron ionization mass spectra of (a, top) phosphoric acid at 95 "C and (b, bottom) phosphorous acid at 70 "C.
(Y+)have been unknown. This is in part due to the low volatility of 3 for which, to our knowledge, there have been no gas-phase data available. We found that phosphoric acid can be evaporated directly to the ion source to give a partial pressure estimated at < Torr at 95 "C. A significant portion of the sample remains as an involatile residue, possibly a polyphosphoric acid. While the ion current of 3'+ from direct ionization of phosphoric acid in the gas phase is too weak for an +NR+ spectrum to be measured, the 70-eV mass spectrum of 3 confirms the stability of the molecular ion, which is identified by its isotope pattern, namely, the lack of satellites at d z 99 (Figure 2a). On dissociation 3'+ forms H2P03' ( d z 81, loss of OH), while elimination of water is unimportant. Further fragmentation gives mainly P02+ (dz 63) and PO+ ( d z 47). These ions are also present in the +NR+ spectrum of 1+, and their formation can in part be due to dissociation of reionized 3'+. We have further investigated the peculiar absence of loss of OH' from 1 and the very low relative abundance of reionized H2O'+. In general, a low relative abundance of a reionized fragment in the NR mass spectrum can be due to several factors, e.g., (1) its inefficient formation from the neutral precursor or its reionized counterpart, (2) low inherent stability resulting in fast neutral or ion dissociations, (3) low reionization eff i c i e n ~ y , ~and ~ . ~(4) ' scattering losses for low-mass fragments formed from high-mass precursor^.^^ As expected from thermochemical both the H2Wf ion and the water molecule are very stable species under +NR+ conditions, as confirmed by the CH3SSCH3/02 +NR+ spectrum which shows a dominant peak of reionized H*O'+ (69%Z+NR+). The reionization efficiency of H20 ( a R ) cannot be measured separately from its with our present apparatus. ion neutralization efficiency (aN) The total +NR+ efficiency of H20'+ (I&" = CLNaR) at 8 keV is relatively large and comparable to that of oxygenated phosphorus ions.33 However, the reionization efficiency of water can be diminished at lower collision energies, such as the 1490 eV for H20 formed from 1, and may also depend on the neutral fragment internal en erg^.^^.^^ The elimination of water and other dissociations of 1 were further investigated with the labeled radicals la-ld. The presence of deuterium in la-ld brings about quantitative changes in the +NR+ spectra (Figure IC, Table 1). The NR
spectrum of P(OD)4 (la) shows a weak but detectable peak of P(OD)3'+ at m/z 85 (0.1% Z+NR+) that persists on neutral collisional activation as shown by the +NCR+ spectrum (Table 1). Both the +NR+ and +NCR+ spectra of l a exhibit significant peaks of reionized D2O at m/z 20 (3.6% X+NR+). This is very different from the spectrum of 1 (Figure lb) which showed a very small peak of H20. It should be noted that the +NR+ efficiencies at 8 keV of D 2 0 + and H20" are almost identical as are the kinetic energies of the neutral molecules formed from 1 and la, respectively. Very similar reionization efficiencies should therefore be expected for the neutral molecule^.^^ Hence the different relative abundances of H20" and D20'+ in the +NR+ spectra must be due to kinetic factors in neutral or ion dissociations. The mixed (H,D)-isotopomers lb-ld show varying relative abundances of reionized H20'+, HDO'+, and D20'+, the first of which overlaps with the isobaric OD+ (Table 1). After corrections for the presumably statistical distribution of hydrogen and deuterium in the water formed, the relative intensities of H20'+, €€Do'+, and DzO'+ indicate that elimination of water increases with the increasing number of deuterium atoms in 1-ld. Although this implies an inverse isotope effect in water formation, the interpretation of this phenomenon is not straightforward because of several possible dissociation paths leading to H 2 0 f , e.g., elimination of water on CAD of precursor l+-ld+ ions and loss of water from intermediate phosphorus radicals, followed by reionization. It should be noted that the presence in la-ld of deuterium has a very small effect on the relative intensities of reionized (H,D)3P04'+, which do not differ much from the relative intensity from 1 (Table 1). Also, the +NR+ spectra show no detectable survivor ions for la+-ld+. Loss of OH' from 1 by direct cleavage of one of the P-0 bonds should form trihydroxyphosphine, P(OH)3, 4. Molecule 4 represents an unknown species, while its isomer, phosphorous acid, O=PH(OH)2 ( 5 ) is a stable compound in the solid state or aqueous solution.s We generated the P(OH)3*+and P(OD)3'+ (4a'+) cation radicals by dissociative ionization of triethyl phosphate and triethyl-& phosphate, respectively. The +NR+ spectra of 4'+ and 4a'+ (Figure 3a,b) give survivor ions of moderate relative intensities, 5.0% and 3.5% Z+NR+, respectively. This suggests that both 4 and 4'+represent stable species under +NR+ conditions so that they should have been detected had they been produced by dissociations of 1. The +NR+ spectrum (Figure 3a) of 4'+ further shows dissociations by sequential losses of OH and water. The complementary ion from reionization of OH' also appears in the spectrum (Figure 3a.b). By contrast, loss of H from 4 (or D from 4a) is insignificant in +NR+ as evidenced by the small intensity ratios, [ d zSl]/[m/z 821 = 0.07 and [ d z 83]l[m/z 85 ] = 0.05 (Figure 3). These fragmentations are consistent with the P(OH)3 bond connectivity, namely, the inefficient loss of hydrogen suggests strong 0 - H bonds and the absence of a P-H bond of a low dissociation energy.35 The CAD spectra of 4'+and 4a'+ show major fragment ions due to losses of OH and OD, respectively, elimination of water, and formation of POH+, PO+, and H30+.36 The relative intensities in the CAD spectra36 of (M - OH)+ and (M H20)'+ differ from those in the +NR+ spectra (Figure 3a,b). The H30+ rearrangement ion which is formed from 4'+by CAD is weak in the +NR+ spectrum, suggesting that this dissociation is not competitive after reionization. Note that H30 does not represent a stable neutral intermediate12dto be detected following reionization. The low relative abundance of H30+ in +NR+ is consistent with the high relative abundances of the products of deep decomposition, e.g., PO+ and P+, that indicate substantial excitation in reionized 4'+ and/or participation of neutral dissociations as discussed above. The CAD and +NR+ spectra
Tetrahydroxyphosphoranyl Radicals
J. Phys. Chem., Vol. 99, No. 8, 1995 2283
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47
Y
Figure 3. +NR+ (CHsSSCH3 (70%T)/Oz (70%T))spectra of (a, top) P(OH)3'+, (b, upper middle ) P(OD),'+, (c, lower middle) HPO(OH)$+. (d, bottom) +NCR+ (CH3SSCH3 (70% T)/He (50% T)/Oz (70% T)) spectrum of P(OH)3'+.
of 4'+ also differ in the relative abundances of P(OH)2+ and P02H'+, with the +NRf spectrum showing a much higher [PO2H]*+/[P(OH)2]+ratio. This is probably due to the greater stability of the neutral 0-POH molecule as opposed to the P(OH)2' radical. A kinetic preference for the formation of P02H'+ by water elimination from reionized 4'+ would be inconsistent with the suppression of other rearrangement dissociations as discussed for H30+. The CAD spectrum of 4'+ and the ion formation by multiple hydrogen transfer rearrangementsw do not exclude coformation of isomeric structures. In order to assess the contribution of other H3P03 isomers these must be prepared independently and
their spectra compared with that of 4. Phosphorous acid (5) represents a stable valence bond isomer of 4, and efforts were therefore focused on generating the phosphorous acid cation radical S+ to examine its +NR+ spectrum. Direct electron impact ionization of gaseous 5 turned out to Torr) vapor be difficult, as the acid showed very small ( e pressure at room temperature, while on slow heating to 70 "C it gradually decomposed by disproportionation to produce phosphine, phosphorus, and phosphoric acid which give the ions in Figure 2bS3' The substantial [ d z 81]/[dz 821 ratio in the part of the spectrum that can be assigned to 5 indicates a facile loss of hydrogen from S+. However, the latter ratio increased by -50% in the course of 5 evaporation, possibly due to contribution of ionic fragments from 3'+ at d z 81 (Figure 3a). We found, however, that dissociative ionization of triethyl phosphite at 70 eV generates an abundant H3P03'+ ion whose CAD and +NR+ spectra differ from those of 4'+,suggesting that a different isomer is formed. The CAD spectrum of the latter36 shows a somewhat higher [HPO]'+/[PO]+ ratio and a lower [PO]+ than does 4, although the differences are rather modest.36 However, in the +NR+ spectrum (Figure 3c) the new H3P03 isomer shows a much lower relative abundance of the survivor ion (0.9% C+NR+) and a significantly larger [ d z 8111 [ d z 821 ratio (0.33) than does 4, while the relative intensities of P+, PO+, and H,P02+ are similar in both spectra. The latter fragment ions are common for the +NR+ spectra of oxygenated phosphorus ions and do not carry structure information for isomer differentiation." The facile loss of hydrogen atom is consistent with the presence in 5'+ of a weak P-H bond, as corroborated by ab initio calculations (Table 2, see below). Interestingly, the +NR+spectra of H3P03'+ formed from triethyl phosphite at 25, 18, and 15 eV show increasing relative intensities of survivor ions and decreasing [ d z 81]/[dz 821 ratios, such that the +NR+ spectrum of H3P03'+ prepared at 15 eV closely resembles that of 4'+. This contrasts the behavior of 4'+prepared from triethyl phosphate, which gives very similar +NR+ spectra at 70 and 15 eV. The energy-dependent formations from triethyl phosphite of 4'+ and S+ indicate opposite kinetic and thermodynamic effects in the precursor ion dissociations. By analogy with dissociations of ketone and enol cation radicals,38one may consider the formation of a thermodynamically less stable isomer to be kinetically preferred in fast dissociations taking place in high-energy ion precursors, while the thermodynamically more stable isomer is preferred in dissociations of low-energy ion precursors, indicating that S+ is less stable than 4'+. This is in accord with ab initio calculations that show significant destabilization of 5'+ against 4'+, as discussed below. In contrast to the ion behavior, collisional activation with helium of neutral 4 at 8 keV results in a decreased survivor ion abundance (1.0% F N R + ) and an increased [ d z 81]/[dz 821 ratio (0.38) in the +NCR+ spectrum (Figure 3d). This indicates isomerization of 4 to 5 on excitation of the intermediate neutral, in accordance with the calculated relative stabilities of the neutral molecules (Table 2) that show 5 to be the more stable 5 isomerization upon collisional isomer. The observed 4 activation is analogous to the gas-phase tautomerization of neutral vinyl alcohol to the more stable a ~ e t a l d e h y d ethat ~ ~ also proceeds as a unimolecular reaction. Collisional activation of neutral 5 also results in an increased relative abundance of HPO3'+ at d z 80 (Figure 3d). This fragment is likely to be formed by subsequent dissociation of H2PO3' under the multiplecollision conditions used for neutral activation. Consistent with this conclusion, the +NR+ spectrum of H2P03+, prepared by dissociative ionization of (C2H50)3P, shows a 1:1 ratio of [H2PO# and [HP03]+.
-
2284 J. Phys. Chem., Vol. 99, No. 8, 1995
TureEek et al.
TABLE 2: Total Energies of Oxygenated Phosphorus Species ~~~~
~~
species
~
symmetry (elec state)
MP2(FULL)/ MP2/ 6-31+G(d) 6-31+G(d) -643.392 097 -643.365 418 -643.393 439 -643.366 753 -643.317 808 -643.317 919 -643.352 400 -643.324 609 -643.352 818 -643.325 031 -642.901 346 -642.873 621 -567.797 883 -567.774 861 -567.726 018 -567.812 186 -567.788 865 -567.751 445 -567.812 249 -567.789 175 -567.708 058 -567.684 866 -567.176 466 -567.153 410 -567.178 582 -567.155 519 -492.096 764 -492.078 030 -492.099 167 -492.080 429 -643.213 396 -643.185 392 -643.080 294 -643.170 018 -643.142 788 -642.508 222 -642.480 540 -642.509 909 -642.482 2 18 -642.401 649 -642.408 982 -567.474 024 -567.450 765 -567.475 511 -567.452 245 -567.404 021 -567.406 224 -567.416 186 -567.393 047 -567.417 897 -567.394 748 -567.358 575 -567.360 363 -567.360 438 -567.362 182 -566.888 573 -566.865 109 -76.212 295 -76.209 776 -75.531 991 -75.529 183 -75.530 825 -0.498 233
total energy" MP4(SDQ)/ MP2/ 6-31+G(d) 6-311+G(3df,2p) -643.384 801
AMP4(SDQ)/ 6-311+G(3df,2p) ZPVE' H298- Hod 151.5 19.9
-643.335 626 127.2
20.2
196 847 405 419 490 107 313
118.6 109 107.5 109.9 107 109.9 97.1 81.2
20.6 17.3
-492.346 039
68.3
14.5
-643.341 277 -642.888 -567.795 -567.746 -567.803 -567.766 -567.804 -567.695 -567.165
126 572 248 939 234 234 747 913
-492.097 585 -643.204 -643.098 -643.158 -642.507
-568.126 485 -568.087 617 -568.149 331 -568.1 10 630 -568.149 431 -568.046 226 -567.497 810 -567.500 031 -492.326 484 -492.329 686
-568.147 -568.107 -568.164 -568.125 -568.164 -568.057 -567.510
881 833 513 369
16.6 16.6 16.6 16.3
150
19.6
150.6 118.4
20.6 19.2
107.5
17.5
-642.408 413 -567.468 402
-567.805 278 -567.806 837
-567.822 915
-567.422 271 -567.420 602 -567.386 514
109 -567.743 -567.744 -567.713 -567.715
002 997 163 215
-567.770 557
107
-567.741 102
109.9
-567.21 1 026 -76.317 971 -75.617 370 -75.619 352
-567.218 805 -76.325 852 -75.631 200
85.7 53.7 21.5
16.1
-567.388 316 -566.872 888 -76.217 657 -75.543 013
-0.499 929
14.2 9.9 10.1 6.1
['In hartrees: 1 hartree = 2625.5 W mol-I. For open-shell species the upper lines give spin unprojected energies, and the lower lines give total = 0.75 (Schlegel, H. B. J. Chem. Phys. 1986,84,4530). AMP4(SDQ)/6-31l+G(3df,2p) energies after annihilation of higher spin states to give (9) = MP4(SDQ)/6-31+G(d) {MP2/6-311+G(3df,2p) - MP2/6-31+G(d)}. Zero-point vibrational energies scaled by 0.89, kJ mol-'. 298 K enthalpy corrections, W mol-'. e I: Single-point calculations on MP2(FULL)/6-31+G(d) optimized ion geometries. f N: Single-point calculations on MP2(FULL)/6-31+G(d) optimized neutral geometries.
+
SCHEME 3 OH
Neutralization of protonated trimethyl phosphate (a+) and its &-methyl isotopomer 6a+ results in extensive dissociation (Figure 4a,b). Of several possible dissociation pathways (Scheme 3) some have been identified tentatively on the basis of deuterium labeling and the known fragments appearing in the CAD spectra of 6+, OP(OCH3)3'+, and HOP(OCH3)2'+ 24c and in the +NR+ spectra of P(OCH&*+ and its fragments." The fragments observed in the +NR+ spectra of 6 and 6a (in parentheses) are underlined in Scheme 3.
Loss of hydrogen from the OH group in 6 forms trimethyl phosphate whose cation radical is known to dissociate by eliminating CH20 to give an abundant fragment ion at d z 1lo.*& In the +NR+ mass spectrum of 6+ neither the reionized trimethyl phosphate nor the fragment at d z 110 survive. The formation of the latter is indicated by the subsequent loss of methyl to give O=P+(OH)(OCH3) which is detected at d z 95. This sequence is corroborated by the mass shifts in the +NR+ spectrum of 6a which shows the corresponding deuterated fragments at d z 117 and 99. Loss of OCH3 from 6 to give HOP(OCH3)2 is not clearly recognizable in the +NR+ spectra and probably represents a minor dissociation route. The peak of HOP(OCD3)2 ( d z 116) from 6a is absent, as is that of the secondary fragment O=P(OH)(OCD3)at d z 98, while the peak of reionized CDsOPOH+ is small. The latter fragment, both radical and ion, is known to be stable under +NR+ conditions." The inefficient loss of CH30 from 6 is somewhat surprising in view of the intense peaks of reionized OCH3 at d z 31 (overlap with P+), its dissociation products at d z 30 (CH20'+) and 29(CHOf) (Figure 4a), and reionized CD30 and its dissociation products at d z 34, 32, and 30 (Figure 4b). One may infer that the methoxy radicals formed originate from dissociations of second-genera-
Tetrahydroxyphosphoranyl Radicals
-
a Td D2 distorted tetrahedron (Figure 5). Ion 1' is substantially stable against dissociations by loss of OH' and H' and elimination of water as shown by the MP4(SDQ)/6-3 1SG(d) ZPVE relative enthalpies (eqs 2-4).
47
I
29
+
95
I
J. Phys. Chem., Vol. 99, No. 8, 1995 2285
P(OH),+
-
P(OH),'+
+ OH'
AHr,o= 487 M mol-'
(2)
AH^^^^, = 495 M mol-' P(OH)C
-
O=P(OH)>
P(OH),+
J61 J 63
,
30
,
,
50
,
,
,
1
,
90
,
110
,
,
130
,
-
AHr,, = 502 kJ mol-'
H3P0,'+ -t- H'
(4)
AHr,298= 508 kJ mol-'
l !
110
70
AHr,, = 290 kJ mol-' (3) AH,,,,, = 294 kJ mol-'
?9
47
1
+ H,O
,
150
m/r
Figure 4. +NRf spectra of (a, top) (CH30)3POHf and (b, bottom) (CD30)3POH+. Collision conditions as in Figure 3a-c.
'
tion neutral intermediates as also observed previously.' Note that CH30+ does not represent a stable structure to be formed by ion dissociations and that the "survivor" (C,H3,0)+ species are likely to correspond to the more stable CH20H+ formed by ion rearrangement.,O Loss of OH from 6 gives rise to the molecule of trimethyl phosphite which is indicated by eliminations of CHO and CH30, the former probably occurring after reionization.,& The fragments show the expected mass shifts on labeling (Scheme 3). Loss of methyl from 6 is weakly indicated by the fragments at m/z 97 and 115 in the +NR+ spectrum of 6a whereas the corresponding nondeuterated fragments are either absent ( d z 109) or of very low abundance ( d z 94). The low abundance of reionized CH30H'+ from 6 and CD30D'+ from 6a indicates the low extent of CAD in the neutralization of 6+ and 6a+, respectively, as elimination of methanol is the dominant dissociation pathway of 6+ on CAD.24c Moreover, the absence of CD3OH'+ in the +NR+ spectrum of 6a strongly indicates the absence of isomeric structures, e.g., O=P(OCD3)2--O+(H)CD3 from protonation at one of the methoxy groups, which would eliminate HOCD3 in contradiction to the spectra. This confirms that mildly exothermic protonation with CdH,+ of trimethyl phosphate (APA = 85 kJ mol-'),' occurs at the P=O oxygen atom, as assumed in previous s t ~ d i e s . ~ , ~ , ~ Ab Initio Relative Energies. In order to interpret the experimental data, we carried out ab initio calculations for several neutral and ion species relevant to the system under study. Previous systematic ab initio studies of hydroxyphosphoranes by Cramer and co-workers revealed the tremendous complexity of the potential energy surfaces of neutral phosphoranes that involve a large number of local minima corresponding to 0-H rotamers and isomers differing in the configuration about the phosphorus atom.6 We have not investigated the details of the potential energy surface for interconversions of 1 and its isomers but rather focused on the dissociation channels and Franck-Condon effects resulting from vertical neutralization and ionization. MPZ(FULL)/6-3l+G(d) geometry optimizations find a local minimum for ion 1+ as
The thermodynamically least endothermic loss of water requires hydrogen transfer between two hydroxy groups that can proceed through a four-membered transition state or via a [P(OH)3* .OH]+ ion-dipole complex. Both these mechanisms are likely to involve substantial activation energies42providing additional kinetic stabilization for structure 1'. The direct cleavage dissociations (eqs 2 and 4) are comparably endothermic, with the loss of OH being slightly preferred. The unimolecular dissociations of 1+ (Figure la) show competitive losses of H and OH, although the former dissociation is slightly preferred. This small discrepancy between the calculated threshold energies and the experimental branching ratio may be due to an activation barrier in the P - 0 bond dissociation, similar to that calculated for dissociations of neutral 1 (see below). The optimized structure of the O=P(OH)20H2+ isomer (2+) shows a long P-OH2 bond (Figure 5), suggesting more facile dissociation by loss of water (eq 5). Although 2+ is destabilized against 1+ by 123 kI mol-' at 298 K, it may represent an isolable structure provided there is a significant isomerization barrier 2+ isomerization barrier separating both isomers. The 1+ has not been studied in this work; we note that analogous hydrogen migrations between geminal hydroxy groups in gasphase cations and cation radicals have energy barriers '200 kJ mol-1.43
-
(HO),PO(OH,)+
-
O=P(OH)> 4-H,O
AHr., = 167 kJ mol-' (5) AH,,298= 171 kJ mol-'
From a different point of view, structures l+and 2+ represent two different products of protonation in phosphoric acid (3) (eq 6). The greater stability of 1+ indicates that the oxo oxygen atom is the most basic site in 3 leading to the most exothermic protonation. The calculated total energies of 1+ and 3 give the proton affinity of 3 as 807 kJ mol-' (eq 6 ) .
H,PO,
+ H+ -
P(oH),+
AH,,^ = -800 k~ mol-'
(6)
= PA = 807 kJ mol-' Vertical neutralization of If creates radical 1 with an initial geometry, which is unstable and on gradient optimization collapses to a local minimum of C, symmetry shown in Figure 5b. Structure 1 represents a stable phosphoranyl isomer with D2
2286 J. Phys. Chem., Vol. 99, No. 8, 1995
TureEek et al.
11.870
9 01P0302 = 149.4
= 76.7
105.3 1.550
9 03P0102=173.7 9 04P0201= 95.4 9 HlOlPO4= 0.2 9 H303P04= 180.0
TS1 Figure 5. MP2(FULL)/6-31+G(d)optimized geometries of (a, top) 1' and 2'and (b, bottom) 1 and TS1. Bond lengths in all dihedral angles given as absolute values.
one hydroxyl substituent and the singly-occupied molecular orbital (SOMO) at phosphorus both occupying the apical positions. Two other stable P(OH)d isomers, one with an apical and the other with an equatorial SOMO, have been characterized recently and found to have comparable stabilities.6a Structure 1 is calculated to be metastable with respect to dissociation by loss of hydrogen (eq 7), and elimination of water (eq 8), but stable with respect to loss of OH' (eq 9).
-
P(OH),'
H,PO,
+ H'
= -37 kJ mol-'
(7)
= -30 kJ mol-' P(OH),'
'OP(OH),
+ H,O
AH,,o= -13 kJ mol-' (8) AH,,,,, = -7 kJ mol-'
P(OH),'
-
P(OH),
+ OH'
AH,,, = 100 kJ mol-' (9) AH,,,,, = 108 kJ mol-'
The dissociation enthalpies of 1 are comparable to those calculated at a slightly different level of theory for the most
A, bond angles in deg,
stable P(OH)d isomer with an equatorial SOMO, e.g., -38 and 130 W mol-' for loss of H and OH at 0 K, respectively.6a In order to assess the kinetic stability of 1 we investigated its dissociation paths and the corresponding potential energy barriers for cleavages of the equatorial O( 1)-H( 1) bond, the axial 0(4)-H(4) bond, and the axial P-0(4) bond (Figure 7). In these UHF/6-3 l+G(d) calculations the pertinent bond was stretched in steps, while the other intemal coordinates were fully optimized. Dissociation of the O( 1)-H( 1) bond reaches an early transition state (TS1, v = 2339i cm-'), which was located by UHF/6-3 l+G(d) calculations and then reoptimized with MP2(FLJLL)/6-31+G(d) to give the geometry in Figure 5. The MP4(SDQ) activation enthalpy, including corrections for ZPVE and 298 K heat contents, is calculated as 90 W mol-' relative to 1. Partial dissociation of the O( 1)-H( 1) bond in the transition state is accompanied by substantial shortening of the P-0 bonds and reorganization of bond angles. Figure 6 shows that the TS1 reaction system undergoes pseudorotation along the 1 coordinate such that 0(1) and O(3) are interchanged, and the 0(2)PO(1) bond angle opens up as the former axial oxygen O(4) moves away from O(1). Mulliken population analysis of the 6-31+G(d) wave function shows the 18a' SOMO in 1 developing into the SOMO in TS1 (Figure 6). Within the basis set used, both SOMOs are constructed predominantly from the s
-
J. Phys. Chem., Vol. 99, No. 8, 1995 2287
TetrahydroxyphosphoranylRadicals Z
Z
I I I
I I
: 8
I
I
--
H
H
.
I I
e
w---
H
X
. . ..
I I
I I I I I I I
I I
TSI (SOMO)
1 (18a') Figure 6. Correlation of SOMO's in 1 and the transition state for O(1)-H( 1) bond dissociation.
0
N
8r
a
5-
-J
Oi
Figure 7. Potential energy profiles along the dissociation paths for O(1)-H( 1) bond cleavage (0)UHF/6-3l+G(d) and (0)UMp;?(FULL)/ 6-3l+G(d), 0(4)-H(4) bond cleavage, (A) UHF/6-3 l+G(d), and P-0(4) bond cleavage (0)UHF/6-3 1+G(d). The disconnected points correspond to relative energies of products at infinite separation. and p diffuse functions on phosphorus and oxygens, whereas the d-orbital coefficients are small. Dissociation of the axial H(4)-O(4) bond follows a steep energy gradient which shows an inflection point at an O(4)H(4) bond length around 1.35 (Figure 7). The calculated potential energy, e.g., 209 kJ mol-' relative to 1at r(0-H) = 1.35 suggests that the 0(4)-H(4) bond dissociation cannot compete with the more favorable O( 1)-H( 1) bond cleavage. The saddle point for this bond dissociation was not located, in part because of SCF convergence failures at 0(4)-H(4) distances > 1.45 A. It is noteworthy that in contrast to the reaction path for the O( 1)-H( 1) bond dissociation, stretching of the 0(4)-H(4) bond is accompanied by very small changes in the P - 0 bond lengths and angles, such that the PO4 framework in the intermediate structures along the latter reaction path resembles that in 1. The different reaction paths for the 0 - H bond dissociations indicate large stereoelectronic effects due to the different orientations of the SOMO and the 0 - H bond being broken. Stabilization is indicated for the transition state in which there is strong interaction between the sp hybrid orbitals at the oxygen
A
A,
reaction center and the phosphorus atom. Although we did not investigate the reaction path starting with elongation of the 0(3)-H(3) bond, the structure of TS1 suggests that the stereoelectronically allowed paths converge in a single transition state. It should be noted that there may be other allowed (lowenergy) dissociation paths that would involve Berry pseudorotation in 1to form an isomeFa that would lose hydrogen through another stereoelectronically assisted transition state. The reaction path for the endothermic loss of OH from 1 was investigated for the cleavage of the axial P-0(4) bond. Stretching of the latter bond is accompanied by a significant potential energy increase (Figure 7). The potential energy surface shows an inflection point and a curvature toward a saddle point. Unfortunately, at P-0(4) distances greater than 2.45 A SCF iterations consistently failed to converge even with the quadratic convergence method, l8 which made it impossible for us to locate and characterize the transition state. In spite of these difficulties, the ascending part of the potential energy surface along the reaction path suggests a substantial activation barrier which we estimate at > 160 kJ mol-' above 1. The kinetic stability of 1 following vertical neutralization depends on the critical energies for neutral dissociations and the internal energy of 1. When only the 2A' ground electronic state of 1is considered, its internal energy consists of the thermal energy carried over from the precursor ion 1+ and the vibrational energy deposited in the electron transfer step, mainly through Franck-Condon effects. The precursor ion internal energy is limited by the threshold for the lowest-energy ion dissociation, which is much higher than that in neutral 1, but the internal energy distribution in the ion is unknown. The magnitude of the Franck- Condon effects can be estimated from the difference in the total energies between the relaxed structure of 1 (corresponding to all v = 0) and that formed by adding an electron to the relaxed geometry of 1+ (Tables 2 and 3). At the MP4(SDQ) level (Table 2) the excitation energy is calculated as 129 kJ mol-', which exceeds the energy barrier for the loss of hydrogen from 1but is insufficient to overcome the barrier for the loss of OH. Vertical neutralization of vibrationally excited 1+ will lead to a spread of the excitation energy in 1, as both P - 0 bond stretching and 0-P-0 bond deformation in 1+ could result in partial relaxation in 1formed by vertical neutralization. In the extreme, neutralization of a vibrationally hot 1+ may produce 1 close to its equilibrium geometry. However, this would require -280 kJ mol-' to be localized in the nine P-0 stretching and bending modes in 1+, as estimated
2288 J. Phys. Chem., Vol. 99, No. 8, 1995
TureEek et al.
TABLE 3: Ionization and Recombination Energies
species P(OH)4 H1p04
MP2(FULL)/ 6-3l+G(d) 4.85
IE(a)b IE(v)' REWd E(a)
10.65
W V )
P(OH)3
8.80
IE(a) W V ) W V )
HPO(OH)2
10.75
Wa)
IE(v)' W V )
PO(OW2
IE(a)
7.94
MP2/ 6-31+G(d) 4.88 7.76 3.60 10.65 12.64 8.80
10.09 7.49 10.74 11.71(11.67)' 9.75 7.95
ionization energy" MP4(SDQ)/ MP2/ 6-31+G(d) 6-311+G(3df,2p) 4.88 7.78 3.56 10.36 13.05 8.89 8.72 10.16 7.56 7.68 11.03 10.40 11.36(11.32)' 11.87 9.41 10.00 7.91 8.02
AMP4(SDQ)/ 6-31l+G(3df,2p)
8.81 7.75 10.68 11.52 9.66 7.98
In units of eV. Adiabatic ionization energy. Vertical ionization energy. Vertical recombination energy of the ion. e Vertical ionization of the C, neutral structure. /Vertical ionization of the C1 neutral structure.
3 H202P01= 144.2
4 02P01O4= 124.1 4 03P0104=-118.2
3 H303P01 = 71.8 - 4 H404P01= 55.1
1.606
9 03P0,02 = 120
3P3)
3+'
Figure 8. Optimized geometries of 3 and 3'+. from the energy difference between the relaxed geometry of 1+ and that formed by vertical ionization of relaxed 1 (Table 3). Hence the calculations indicate that a large fraction of 1 formed by vertical neutralization is kinetically unstable and will dissociate to H3P04 in accordance with the +NR+ spectrum. The P-0 bond dissociation in 1 leading to loss of OH' becomes competitive only when the energetically more favorable loss of H is slowed down by primary deuterium isotope effects, as observed for la. The calculated relative energies further suggest that dissociations of H3P04 (3) formed from 1 should occur after reionization. 3 is a very stable molecule whose P-0 bond dissociation energy, calculated as 460 kJ mol-' at 298 K, exceeds substantially the internal energy available for 3 from threshold dissociation of 1, e.g., 159 kJ mol-' from the dissociation exothermicity and internal energy excess. As discussed above, neutralization of hot but stable 1+ of up to -300 M mol-' internal energy (eq 3) can result in vibrational excitation in 1 which is then channeled to 3 and further to reionized 3'+. The P-0 bond dissociation energy in 3'+ (eq 10) is much lower than that in neutral 3, so that a significant fraction of vibrationally excited but stable 3 becomes unstable following reionization. This may explain the low relative abundance of
H3P04'+
-
O=P(OH),+
+ OH'
AHH,,, = 2 18 kJ mol-' (10) AH,,,,, = 223 kJ mol-'
reionized 3'+ in the NR spectrum of 1+. By contrast, vertical
ionization of thermal 3 forms a significant fraction of stable cation radicals 3'+ as evidenced by the electron impact mass spectrum that shows H3P04'+ ions of > 100 ,us lifetime (Figure 2a). While the enthalpy carried over from thermal 3 to 3'+ is low (e.g., 27 kJ mol-' at 373 K), vibrational excitation can be provided by large Franck-Condon effects in the vertical ionization of 3 (Table 3). The MP4(SDQ) calculations of Table 2 give the energy difference between the vertically ionized and relaxed 3'+ as 259 kJ mol-'. This value appears to be exaggerated, as it would make the ion kinetically unstable; MP2 calculations give an energy difference of 192 kJ mol-' in somewhat better qualitative agreement with experiment. A potential energy barrier in the P-0 bond dissociation in 3'+ would also increase the kinetic stability of the ion. Provided that the Franck-Condon effects in collisional reionization and electron impact ionization are similar, the different stabilities of 3'+ observed in the NR and E1 mass spectra reflect the very different vibrational energies of the neutral molecules. The Franck-Condon effects in vertical ionization are mainly due to the large differences in the P=O and P-OH bond lengths in 3 and 3'+ (Figure 8). In neutral 3 the P-O( 1) bond is much shorter than the P-OH bonds, such that the former has a substantial double-bond character.44a By contrast, ionization in 3'+ results in considerable shortening of the P-OH bonds while the P-O( 1) bond is elongated to the extent of becoming a single bond (Figure 8). Mulliken population analysis of the UHF/631+G(d) wave function in 3'+ shows an almost complete spin localization at 0(1) (spin density 1.00), while the phosphorus atom carries a large positive charge (+2.4). It should be noted that other rotamers of 3'+ may possibly exist as stable equilibrium structures by analogy with the rotamers of 3,44abut
TetrahydroxyphosphoranylRadicals 101.2
J. Phys. Chem., Vol. 99, No. 8, 1995 2289
92.9
108.3
4 OPOO = 117.2 4 HOP0 = 177
4+' c3
4 O1P0203= 1 17.4 3 HjP0203 = 179.8 3 H2QPO3 = 170.7 117.3
115
TS(5 - 4 ) C,
118.5
120
3O1PO2O3= 121.3 3 HI PO203 = 1 16.4 3 H202P03 = 72.5
5 Figure 9. Optimized geometries of 4, 5, 4'+, and S+.
those were not investigated. Very similar changes in the P=O bond length and spin density distribution are found for ionization S+,Figure 9) whose cation radical of phosphorous acid (5 (S+)also shows a long P-O(1) bond and spin localization at o(11.45 The existence in these cation radicals of localized radical centers at the oxo oxygen atoms provides a chemical rationale for the behavior of the related alkyl phosphate and phosphonate cation radicals that undergo facile hydrogen rearrangement to form distonic isomers (Scheme 1).24c The rearrangement is driven by the greater stability of the distonic cation radicals, as estimated from the O-H and C-H bond dissociation energies24c.26,46 and is presumed to have a low activation The present calculations suggest that due to the spin localization at the receptor oxo atom the latter closely resembles the oxygen atom in an alkoxy or hydroxy radical. The latter radicals are known to abstract exothermically hydrogen atoms from C-H bonds with very low activation energies, both for the intra- (Ea and intermolecular reactions (E, = 4 kJ = 45 kJ The hydrogen affinity28 of the oxygen radical terminus can be further enhanced by the electron-withdrawing effect of the positively charged phosphorus atom, resulting in both the greater stabilization of the distonic products and a low barrier to hydrogen transfer.26 In order to interpret the +NR+ spectra of P(OH)3 (4) and HPO(OH)2 (S), the relative energies of 4, 5, 4'+,S+,and their dissociation products were also investigated. The size of these systems allowed us to incorporate energy corrections obtained from single point MP2/6-3 11+G(3df,2p) calculations, similar to those employed in the Gaussian 2 theory,*, such that the relative energies approximate those from MP4(SDQ)/6-311f G -
-
(3df,2p) calculations. The asymmetric structure 4 (Figure 9) represents the global minimum on the potential energy surface of P(OH)3 isomers."" In agreement with previous we find two structures for phosphorous acid 5, with the C1 structure (Figure 9) representing a potential energy minimum and the C, structure (Table 2) being a first-order saddle point for the low-barrier rotation about the P-OH bonds (Y = 46i cm-'). Unimolecular 5 4 isomerization is endothermic (eq 11) and proceeds through a C, transition state (TS5 4 , = ~ 2187i cm-') located 269 kJ mol-' above 5 at 298 K.
-
P(OH),
-
HPO(OH),
-
AHr,, = -44.5 kJ mol-' (1 1) AHr,298 =
-45.2 kJ mol-'
Both 4 and 5 are thermodynamically stable as evidenced by the very endothermic dissociations by P-0 and O-H (or P-H) bond cleavages (eqs 12 and 13). P(OH),
-
'P(OH),
+ 'OH
AHr,, = 3 18 kJ mol-' (12) AHr,298= 325 kJ mol-'
-
O=P*(OH),
+ H'
= 304 kJ mol-' AHr,298
(13)
= 309 k~ mol-'
The optimized structures of the dissociation products are shown in Figure 10. The calculated relative enthalpies show that the isomerization barrier for 4 5, although quite substantial (224 kJ mol-'), is lower than the thermochemical
-
TureEek et al.
2290 J. Phys. Chem., Vol. 99, No. 8, 1995 I
4 OIPOO = 126.6 3 HOPOt = 38
surface accessed by vertical neutralization exceeds substantially the energy barriers separating the stereoisomers (20-40 kT Thus the radical formed has a sufficient vibrational energy to reach any low-energy dissociation channel regardless of the shape of the potential energy surface along the reaction path. Although mapping the entire potential energy surface of P(OH)4' would be a formidable task, the preferential loss of hydrogen from 1 found by experiment indicates that there are no low-energy channels for the competing P-0 bond cleavage originating in other P(OH)4* isomers. Very substantial Franck-Condon effects are also predicted for vertically formed 5, and 5'+ (Table 3). However, since the Franck-Condon energies do not exceed the thermodynamic thresholds for dissociations of these species, neutralization of S+ and reionization of 5 produce fractions of molecules and ions, respectively, that are stable on the microsecond time scale as observed. The interpretation of Franck-Condon effects in the vertical electron transfers 4'+ 4 and 4 4'+is somewhat complicated by the large number of isomers for the neutral system43aand presumably for the ion system, too. Thus vertical neutralization of the C3 structure 4'+ forms a neutral species of 103 kJ mol-' potential energy above the C1 structure 4. Other stable P(OH)3 structures, calculated by Cramer and Gustafson,4?a are within 20 kJ mol-' above 4,and presuming low rotational barriers,6a they should be accessed from the neutralized intermediate. Hence the Franck-Condon effect in neutralization of 4'+ will manifest itself in complete equilibration of the structures on the P(OH)3 potential energy surface. However, Franck-Condon effects alone are insufficient to cause 4 and/ '+ to dissociate. The significant fragmentation observed or 4 upon neutralization-reionization of P(OH)3'+ must therefore be due to neutral and/or ion excitation through collisional energy transfer. 16c,49
-
4 HOP0 = 124.8
c2 Figure 10. Optimized geometries of (P,O,H) dissociation products.
threshold for bond dissociation in 4 and 5 , such that isomerization will be the only reaction channel in 4 at excitation energies below 304 kJ mol-'. The calculated energy barriers are consistent with the experimental data that show 4 5 isomerization on collisional excitation of neutral 4 formed by vertical neutralization (Figure 3d). By contrast, P(OD)3formed by dissociation of l a isomerizes to a much smaller extent on collisional activation (Table 1). This is likely due to a combination of isotope effects on the deuterium migration in P(OD)3 and the lower internal energy of the latter species formed by the endothermic dissociation of la. The order of stabilities is reversed in cation radicals 4"and 5'+ (Figure 9) of which the former is 136 kJ mol-' more stable at 298 K. By analogy with the potential energy surface for neutral P(OH)3 isomers, the surface for P(OH)3'+ cation radicals may involve several minima in addition to the C3" structure 4'+. However, the existence of other P(OH)3'+ would not reverse the generally greater stability of trihydroxyphosphine cation radicals compared with the phosphorous acid isomer. The calculated total energies show a low thermochemical threshold for the loss of the phosphorus-bound hydrogen from 5" (eq 14), explaining the very facile loss of H upon neutralizationreionization of 5'+ (Figure 3c). The large difference in the P-H
-
HPO(OH),'+
-
O=P(OH)2+
+ H'
AHr,o= 115 kJ mol-' (14) AHr,298= 119 kJ mol-'
bond dissociation energies in neutral 5 (353 kJ mol-') and ion mol-' 5'+ (eq 14) strongly suggests that the dissociation occurs following reionization. The distinct +NR+ spectra of 4'+ and S+ further indicate that the isomers are separated by a substantial energy barrier. The energy effects of electron transfer in vertical neutralization and reionization are summarized in Table 3, which gives the calculated ionization and recombination energies for 1, 3, 4,and 5 . The energies show that the chemistries of both 1 and I+ are dominated by very large Franck-Condon effects that make the vertically formed species kinetically unstable. It is worth mentioning that the point on the P(OH)4' potential energy
-
Conclusions This combined experimental and computational study of the &PO4 and H3P03 systems points to the importance of FranckCondon effects on vertical electron transfer in phosphorus oxoacids. Ionization of the intrinsically stable molecules and radicals results in very dramatic changes in the equilibrium geometries and electron distributions that determine the stabilities and reactivities of the ions formed. Conversely, vertical reduction of stable cations results in vibrational excitation in the neutrals formed that can dominate their unimolecular chemistries, as found for P(OH)4*. This finding strongly indicates that large Franck-Condon effects in the vertical formation of radicals centered at second-row elements might be a common p h e n o m e n ~ n . ~ ~ . ~ ~ ~ , ~ ~
Acknowledgment. Support by the National Science Foundation (Grants CHE-9102442 and CHE-9412774) and the donors of the Petroleum Research Fund administered by the American Chemical Society is gratefully acknowledged. Generous computer time allocation was provided by the Come11 National Supercomputer Facility that receives major funding from the National Science Foundation and the IBM Corporation, with additional support from New York State and the Corporate Research Institute. We wish to thank Professor C. J. Cramer and Dr. S. M. Gustafson for helpful discussions and for providing us with the results of their calculations prior to publication. References and Notes (1) University of Washington. (2) University of Wisconsin. (3) Corbridge, D. E. C. Phosphorus; Elsevier: Amsterdam, 1990; Chapter 4, pp 333-346.
J. Phys. Chem., Vol. 99, No. 8, 1995 2291
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