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Proton Affinity and Collision=Induced Decomposition of Ethoxylated Alcohols: Effects of Intramolecular Hydrogen Bonding on Polymer Ion Collision=Induced Decomposition Hung-Yu Lin, Alan Rockwood, M. S. B. Munson, and Douglas P. Ridge' Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Collision-induceddecompositionand ion/molecule reactions of poly(ethy1ene glycol) (PEG) and ethoxylated alcohols (EA) are investigated using Fourier transform ion cyclotron resonance (FTICR) techniques. Self chemical ionization of PEG1000, PEG-400, and EAs produces protonated parent molecules. The protonated parent ions will transfer a proton efficiently to other PEG or EA molecules. The alkali metal adducts of PEG produced by laser desorption transfer the alkali metal ion to other PEG molecules. The CID spectrum of a typical protonated ethoxylated alcohol ( ( C I ~ H ~ ~ ( O C ~ H ~ ) ~ O is Hfound ) H + )to be dominated by cleavage of only three of its 15 C-0 bonds. A mechanism accounting for this behavior is suggested, involving a structure of the protonated EA with an intramolecularstrong hydrogen bond. Measurement of the proton affinity (240.2 kcal/mol) and the entropy of protonation (-32.0 eu) is consistent with an intramolecular strong hydrogen bond. Clustering reactions suggest the intramolecular strong hydrogen bond structure is preferred for (R(OC2H4),OH)H+ where n > 3 (R = alkyl). INTRODUCTION The mass spectrometry of poly(ethy1ene glycol) (PEG) and ethoxylated alcohols (H(CH2),(0C2H~),OH)has been examined, both because these speciesare useful industrial products and because they are useful as models for polymers in general.14 Most ionization methods useful for polymers produce protonated or metalated PEG and ethoxylated alcohol molecules. Protonated polymers are important in polymer mass spectrometry and interesting in their own right. Ethoxylated alcohols, for example, are used as surfactants in biological studiesH and also in detergents, so their interactions with proton donors are of interest on that account. The structure of gas-phase protonated polymer molecules, their reactivity, and their behavior in collisional activation are all important in the mass spectrometry of polymers. We report here on an examination of the structure and reactions of gas-phase protonated poly(ethy1eneglycol) and ethoxylated alcohols including reactions with the neutral polymers. We also consider metal ion-transfer reactions between PEG molecules. In addition to examining proton(1) Lai, s. T. F.; Chan, F. W.; Cook, K. D. Macromolecules 1980,13, 963. (2) Lattimer, R. P. Int. J.Maas Spectrom. IonProcessecr 1984,55,221. (3) Lattimer, R. P.; Hansen, G. E. Macromolecules 1981, 14, 776. (4) Garavito, R. M.; Hinz, U.; Neuhaus, J.-M. J. Biol. Chem. 1984,259, 4264. (6) Haydon, D. A.; Urban, B. W. J. Physiol. 1983, 341,411. (6) Prottey, C.; Fergueon, T. F. M. Food Cosmet. Toxicol. 1976, 14, 426. 0003-2700/93/0385-2 11S$04.00/0
transfer reactions of poly(ethy1eneglycol), we determine the proton affinity and entropy of protonation of a particular ethoxylated alcohol. We then consider how these data reflect on the structure of the protonated molecule. Finally, we show that these structural considerationsprovide a simple rationale for striking features of the CID spectra of the ion. Fourier transform ion cyclotron resonance (FT-ICR)'-S makes these studies possible. The very long ion trapping time available makes it possible to observe ion/molecule reactions of species with very low vapor pressures.
EXPERIMENTAL SECTION Compounds were obtained commercially and used without further purification. The experiments were done using a commercial dual-cell FT-spectrometer (FTMS 2000, Extrel, Madison, WI) equipped with a 3-T superconducting magnet, a COz pulsed laser, and an automated probe assembly. The dualcell Fourier transform ion cyclotron resonance cell was first introducedby Ghaderi and Littlej~hn.~ It is especially useful in MS/MS experiments.10 Ion/molecule reactions can occur in a relatively high pressure cell (source), and ions of interest can then be transferred to the adjacent differentially pumped cell (analyzer). There the ions can react with a different reagent, be subject to CID, or be subject to precise mass measurement at very low pressure. PEG and ethoxylatedalcoholswere introduced into the source region with the solid sample probe. A 70-eV electron beam was pulsed for 5 ms to ionize the sample. Amines were admitted to the source through a batch inlet system after several freeze-pump-thaw cycles to eliminate noncondensable gases. Using the ion-trapping ability of the FT-ICR system, self chemical ionization (self-CI)experimentswere done by trapping the E1 fragments in the source and allowing them to react with neutral polymer molecules for a variable reaction time. Product ions were then mass analyzed and identified. The complete selfCI sequence is illustrated in Figure la. Like all the experimental sequencesused in these studies,the self-CIexperiments are under computer control; they can be repeated any number of times and the resulting spectral signal averaged. For collision-induced dissociation (CID) experiments, ions formed by self-CIwere transferred from the source to the analyzer region. All the ions except the protonated ethoxylated alcohol were then ejected by exciting their cyclotron motion to the point that they strike the cell walls, lose their charge, and disappear from the system. The isolated ion was then excited to a selected cyclotron radius corresponding to a particular kinetic energy before a pulsed valve admitted argon as a collision gas. A spectrumwas obtained after a short delay for collisions to occur. The complete CID sequenceused is illustrated in Figure lb. Under the conditionsused in the present experiments, the excited ion on average collided about 2 times with neutral argon atomsbefore the spectrum was obtained. The number of collisions was taken (7) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,25,282. (8) Marshall, A. G.; Comisarow, W. B. Chem. Phys. Lett. 1974,26,486. (9) Ghaderi, S.; Littlejohn, D. P. Presented at the 33rd Annual Conference on Mass Spectrometry and Allied Topics of the American Society for Mass Spectrometry, San Diego, CA, 1986. (10) Cody, B. R., Jr.; Kinsinger, J. A.; Ghaderi, S.; Amster, I. J. Aml. Chim. Acta 1986, 178,43. 0 lSS3 Amerlcan Chemlcal Society
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(a) I
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Flgure 1. Experimental FT-ICR
sequences: (a) self-CI; (b) CID; (c) laser desorption.
as k J r n ( t )dt, where n(t)is the time-dependent number density of Ar where the valve opens at time t = 0 and a spectrum is obtained at a time t = T. The integrated number density is proportional to the pressure in the gas storage volume (0.4 Torr) that feeds the pulsed valve. The proportionality constant was determined by using known rate constants in calibration procedures described in detail elsewhere." The laser desorption experiments were done with a Tachisto C02 laser focused on the sample deposited on the stainless steel probe tip. The experiment sequence used is shown in Figure IC. For the equilibrium proton affinity study, about 5.0 x 10-8 Torr NJV-dimethylanilinewas introduced to the source with the ethoxylated alcohol (CI~HZ(OCZH~,OH). After a 5-ma pulse of the 70-eV electron beam and a 10-s reaction time, all the ions were ejected except the protonated amine. This ion was then allowed to react further with the neutral molecules present until the relative intensities of the protonated amine and the protonated ethoxylated alcohol did not change with further reaction. The relative peak intensities of the protonated amine and protonated ethoxylated alcohol were taken as proportional to the relative concentrations of the two ions. The ratio of neutral pressures was estimated from the electron impact mass spectrum of the mixture and the relative ionization cross sections of the components. Ionization cross sections were estimated by the method of Georgiadisand Bartmess,12which relates the ionization cross section to the polarizability. The temperature of the cell was regulated by heating elements and thermocouples attached to the vacuum chamber wall. These experimental procedures produced values of the temperature-dependent equilibrium conatants for proton transfer between amines and aliphatic diols in good agreement with those obtained using a pulsed highpressure mass spectrometer.13
RESULTS AND DISCUSSION Reactivity of Poly(ethy1eneglycol): Proton-Transfer Reactions. Lee, Popov, and Allison14reported the E1spectra of several crown ethers and showed that the (OCzH4)14H+ (11) Lin, Y.;Nicol, G.; Ridge, I). P. Presented at the 40th Annual Conference on Maas Spectrometry and Allied Topics, 1992. (12) Bartmess, J. E.;Georgiadis, M. R. Vacuum 1983, 33, 149.
45 89
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Flgure 2. E1 spectrum (70 eV) of EQ-1000 obtalned when the sample was Introduced to the source by a heated probe. The probe temperature was 70 'C. Inset Is a full spectrum to mlz 1000 showing the absence of hlgh-mass peaks.
ions are the dominant fragments with protonated ethylene oxide as the base peak. Oliveira16and Jaeger's reported that the mass spectra of crown ethers consist primarily of peaks corresponding to the successive loss of OCzH4 units. Because poly(ethy1ene glyco1)s also have the ethylene oxide repeating units we might expect similar E1 fragments. The 70-eV E1 spectrum of PEG-1000 is shown in Figure 2. Only low-mass fragments appear similar to the crown ether spectra reported by Lee, Popov, and Allison.14 The dominant ions are at mlz 45,89, and 133corresponding to ( O C Z H ~ ) I ~ H + . A self chemical ionization spectrum was obtained after fragment ions were trapped in the source for 10 s. The spectrum shows a distribution of ions corresponding to the (13) Nicol, G. Thesis, University of Alberta, 1988. (14) Lee, Y. C.; Popov, A. I.; Allison, J. Znt. J. Mass Spectrom. Zon Processes 1983,51, 267. (15) D e S o m a Gomes, A.; Oliveira, C. M. F. Org. Mass. Spectrom. 1977, 12, 407. (16) Jaeger, D.A.;Whitney, R. R.J. Org. Chem. 1975,40, 92.
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of PEG samples. The K+ IDS (K+ ionization of desorbed species) technique reported by Bombickand Allison23involves rapid heating to desorb nonvolatile compounds and then add the potassium ions to the molecules in the gas phase. The K+ IDS spectrum of PEG-1000 shows ions of the (M K)+, and (M + K - H2O)+series that can be useful in determining the average molecular weight of the mixture. In view of the facility of the proton-transfer process, we undertook to determine whether MK+ ions react with polymer neutrals and whether such reactions effect the observed molecular weight distribution. The laser desorption of PEG-1000 with reaction times from 0.05 to 5 s in our study shows little change in the numberaverage molecular weight. The values were 1044,1002, and 983 for the MK+, MNa+ and MH+ ions, respectively. The sample was placed directly on a stainless steel disk mounted on the probe. Ions of the forms (M + K)+ and (M Na)+ are dominant, but (M + H)+ is also observed. PEG-400 behaves similarly, but its vapor pressure is high enough that K+-transferreactions can be shown to occur by an experiment illustrated in Figure 5. The ion a t mass 497 corresponding to [H(OC2H4)loOHlK+was ejected, and it then grew back at long reaction time aa a result of K+-transferreactions. Figure 5a is the spectrum without ejection. Figure 5b is the spectrum taken after ejecting m/z 497, and Figure 5c is the spectrum taken 100 s after ejection. The probe tip was at 60 "C and the disk was presumably at a temperature between 60 OC and the temperature of the vacuum chamber (25 OC). Evidently the vapor pressure of the ethoxylated alcohol is enough a t this temperature for reactions with ions trapped in the cell to occur. The laser pulse could eject some neutrals from the surface, effectively increasing the PEG vapor pressure for a short time, but that brief pressure burst is probably unimportant on the time scale of 100 s. While addition of metal ions to poly(ethy1ene glycol) has been observed, the present results appear to be the first example of metal ion transfer from one large poly(ethy1eneglycol)molecule to another. Thus while polyfunctional molecules such as poly(ethy1eneglycol) are polydentate ligands, transfer of the metal ions from one molecule to another is still quite efficient ( k > 10-12 cm3/s). It has recently been reported that K+(triglyme) and K+(tetraglyme) add to triglyme and tetraglyme, respectively, forming KL2+.24 We do not observe such dimer formation in the PEG-1000 system, but rather we observe K+ transfer as described above. That suggests that K+ transfer is more efficient than association, at least for the PEG molecules we examined. We note that K+ transfer in the triglyme and tetraglyme experiments would not change the mass spectrum and would therefore have been undetectable. Reactivity and Structure of Ethoxylated Alcohols: Conformation and CID. We used ethoxylated alcohols for further studies because they are structurally similar to PEG and the single components are readily available. We undertook to examine the CID of a protonated ethoxylated alcohol, (H(CH2)12)OC2H4)70H)H+, and to determine the proton affinity of the parent neutral. We did this in the hope of learning something about the conformation of the gasphase protonated polymer. The protonated ethoxylated alcohol is formed from reactions of the alcohol with its E1 fragments in the source (selfCI). After a 10-s delay for the self-reaction, the MH+ ion at m/z 495 was transferred to the analyzer to examine ita CID. The MH+ ion was then excited translationally by a short rf pulse at ita cyclotron frequency and allowed to collide with Ar admitted through a computer-controlled pulsed valve. The collision-induced fragments were then mass analyzed and identified.
+
xI21 943 x.22 987
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Figure 9. Self41 spectrum of PEGlOOO. Same conditions as Figure 2, except a reaction time of 10 s was allowed for chemical ionization reactions to occur.
protonated molecular ions for the components of the mixture (Figure 3). The self-CI of PEG-400 after 10-5 reaction time shows a similar distribution of protonated parent ions (Figure 4a). It has been previously observed that protonated polyfunctional ethers capable of forming strong intramolecular hydrogen bonds do not readily dimerize with the parent neutral under ICR conditions.17 This suggests the possibility that proton transfer between such species may not proceed efficiently. T o investigate this, an experiment was performed in which 10 s was allowed for self-CI of PEG-400. The ion at mlz 415 was then ejected (Figure 4b). The remaining ions were then allowed to react further with the neutral PEG molecules present in the cell for 3 s (Figure 4c) and for 30 s (Figure 4d). The ejected peak grows back, indicating that proton transfer between different poly(ethy1ene glycol) molecules occurs quite readily (k > l W 2 cm3/s). The final distribution of ions (Figure 4b) is essentially the same as the initial distribution (Figure 4a), indicating that no particular protonated ion is more reactive than the others. Gas-phase proton transfer has been shown to determine the ions observed in FAB spectra.'* It is therefore significant to demonstrate as we have here the facility of gas-phaseproton transfer between large, polyfunctional molecules, since FAB analytes are frequently large, polyfunctional molecules. Reactivity of Poly(ethy1ene glycols): K+-Transfer Reactions. The molecular weight distribution of a polymer mixture is important in determining its properties. However, it is sometimes difficult to use mass spectrometry to analyze polymer samples due to their involatility and their complex composition. Laser desorption is a soft ionization technique which can simultaneously vaporize and ionize involatile polymers. A mass spectrum then provides polymer molecular weight distribution.lS22 Cotter and LattimerlQreported that laser desorption time of flight mass spectra of PEG and other polymers show quasimolecular ions (M + K)+ and minor fragments. Brown and co-worker@ and Nuwaysir and Wilkins21 reported the laser desorption FTMS mass spectra of PEG and other polymers,ml21 which show a distribution of ions corresponding to (M + Na)+, (M + K)+, and (M + H)+ series. The (M + K)+ ions are frequently dominant and are therefore used to determine the molecular weight distribution (17) Morton, T. H.; Beauchamp, J. L. J. Am. Chem. SOC. 1972,94, 3671. (18) Summer, J.; Moralee, A.; Kebarle, P. Anal. Chem. 1987,59,1378. (19) Cotter, R. J.; Honovich, J. P.; Olthoff, J. K.; Lattimer, R. P. Macromolecules 1986,19, 2996. (20) Brown, R. S.; Weil, D. A.; Wilkins, C. L. Macromolecules 1986, 19, 1255. (21) Nuwaysir, L. M.;Wilkms, C. L. Anal. Chem. 1988,60, 279. (22) Kinsinger, J. A. J. Polym. Sci. Lett. 1985,23, 453.
+
(23) Bombick, D. D.; Alliaon, J. Anal. Chem. 1987,59, 458. (24) Zhang,H.; Deardon, D. J. Am. Chem. SOC. 1992,114, 2754.
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Figure 4. PEG400 spectrum after (a) 10-s reaction tlme; (b) 10-s reaction tlme and then ejectlng 415+; (c) 3 s after ejectlon; and (d) 30 s after Torr from an ionization gauge reading. ejection. The sample pressure is about 3 X
The spectra in Figure 6a,b were obtained at 2.9- and 7.3-eV center of mass energy (40- and 100-eV laboratory energy). The collison gas pressure rises quickly to about 10-6 Torr when the pulse valve is opened under these conditions and then drops back toward 10-8 Torr with a time constant of about 0.1 s. Typically the ions suffer an average of two collisions before the collision products are sampled. Primary fragments are mlz 45,89,133,407, and 451 resulting from the cleavage of carbon-oxygen bonds. Similar fragments were observed in the CID spectrum of a pure ethoxylated alcohol by Mabud et al.25 in a triple-quadrupole experiment. The fragment ions H(OC2H&9+ and complementary ions formed by loss of (OCzH4)1-2 contribute more than 70% of the total fragmentation. This is astriking degreeof specificity. There are 15 C-0 bonds in the chain, and 70% of the fragmentation results from cleavage of just three of them. To explain this specificity we propose the mechanism illustrated in Figure 7. The protonated molecule forms an intramolecular strong hydrogen bond, forming a cyclic structure. Protonation of the OH is probably avoided because an OH oxygen is less basic than an ether oxygen.Z6 On activation the hydroxyl oxygen atom attacks an a-carbon to (26) Mabud, M. A.; Dreifuaa, P. A,; Kdinger, W. E.; Smith, M. W. Presented at the 37th Annual ASMS conference on Mass Spectrometry and Allied Topics May 21-26, Miami, FL, 1989. (26) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984,13, 695.
give stable ions such as 45+ and 89+ (protonated oxirane and dioxane). An exocyclic oxygen atom can attack an exocyclic a-carbon much more readily than an endocyclicoxygen atom can attack an endocyclic a-carbon. Hence, fragments of the type H(OC2H&OH)H+ are not observed at all, and fragments of the type C~~H=(OCZH~),+ are almost completely lacking as well (mlz 301 in Figure 6a is the only exception (n = 3)). The fragments produced by such a mechanism correspond to those which dominate the CID. It is interesting to note that Kiplinger and Bursey,2' in examining the CID of protonated PEG, suggested that the protonated PEG forms an internally hydrogen-bonded species which dehydrates to form a protonated crown ether. Chemical ionization of the ethoxylated alcohol in the ICR using acetone-& as a reagent gas produced fragments with the stoichiometry (C2HdO),H+ (n = 2, 3) rather than (CzH40),D+ consistent with the proposed mechanism. Conformation and Proton Affinity. To help substantiate our proposed CID mechanism and to better understand the properties of ionic polymersin the gas phase, we undertook a study of the proton affinities (PAS) of the ethoxylated alcohols. An equimolar mixture of three ethoxylated alcohols (C12H26(OC*H4)190H)was placed on the probe and the vapor ionized by the electron beam pulse. As is evident in Figure 8, at long reaction times the protonated parent ions, MlH+, (27) Kiplinger, J. P.; Bursey, M. M. Org. Mass Spectrom. 1988, 23, 342.
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Figure 5. (a) Laser desorption spectrum of PEG400 without ejectlon; (b) spectrum taken after ejecting mlr 497 ([H(OC2H4)loOH]K+);and (c) spectrum taken 100 s after ejection.
Flguro 6. Collision-Induceddecompositionspectra of (C12H26(OC2H4hOH)H+ at (a) 2.9- and (b) 7.3-eV center of mass collision energy. The colllslon gas is argon and the average parent Ion suffers approxlmatety two collisions between excltatlonand acquisition of the CID spectrum.
M2H+, and M3H+, appear as product ions (Mx = C12H25' (OC2H4),0H). The dominant ions a t very long reaction time are (MlMzH)+,(MlMSH)+,(M2M3H)+,(M12H)+,M22H)+,and (MS2H)+. The relative intensity of (M32H)+continuously increases with reaction time while other ion intensities level off or decrease. This suggests that PA(CI~HZ~O(C~H~O)IH) < PA(C~~H%O(CZH~O)~H) < PA(C~~H~SO(C~H~O as)SH), H / " 4 suming that those species which form the strongest intramolecular hydrogen bond have the greatest PA. Figure 7. Collision-Induced decomposhlon mechanism. A mixture of R(OC2H4)uOH species gives only (M + H)+ ions at long reaction time. No significant (MaMbH)+ was observed. The total ionization resulting from electron impact on the y = 4-6 mixture was similar to that for the y = 1-3 mixture, suggesting similar total pressure in the two cases. This suggests that a protonated ethoxylated alcoholforms an intramolecular strong hydrogen bond when the size of the resultant ring is large enough (n > 3). Otherwise, the protonbound dimer of two ethoxylated alcohol molecules is formed. The intramolecular interaction is preferred and eliminates the formation of intermolecular proton-bound dimers when the closed ring structure is accessible. We note the similarity of these results to those of Morton and Beauchamp" with speciesof the type CH30(CH2),0CHs. The found that the protonated dimethoxy species with n > 4 did not dimerize and similarly concluded that species that could form a large enough ring would form an intramolecular strong hydrogen bond. .5 1 5 10 30 60 ! I In a similar study, a mixture of three ethoxylated alcohols Source Reaction Time ( 8 ) with different alkyl chain lengths but with the same number Flgure 8. Variation with reaction tlme followlng electron beam pulse of ethylene oxide repeating units was examined. The relative of the relathre peak Intenslties of proton-bound dlmers of ethoxyiated peak intensity of (M + H)+ does not change with reaction alcohols with different number of N 2 H 4 unks W = C12H25(OC2Hl)rOH. time, indicating that their proton affinities are fairly close to The lines are lnterpoletlons through the data. each other. The difference in relative peak intensities mass 494. As shown in Figure 9, the ratio of the relative probably reflects only the relative vapor pressures. intensities of the protonated amine and protonated ethoxNJV-Dimethylaniline was allowed to react with a protoylated alcohol becomes constant at long reaction time. We nated ethoxylated alcohol, Cl2H2s(OCzH4),0H, which has
rs
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, lQB3
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assumed that this represents equilibration between these two ions. This experiment was repeated a number of times a t each of three different temperatures. At each temperature an apparent equilibrium occurred. A van't Hoff plot of the equilibrium constants for the proton-transfer reaction (Figure 10) gives AH = 17.1 kcal/mol and A S = -32 eu for the proton transfer. AH and the known proton affinity of N,Ndimeth~laniline~' gives 240.5 kcal/mol as the proton affinity of the ethoxylated alcohol. A typical straight-chain ether, n-butyl ether, has a proton affinity of 203.7 kcal/mol,26which is about 36 kcal/mol lower than that of the ethoxylated alcohol. This is an appropriate value for astrong hydrogen bond (D(H20.-H30+) = 31.5 kcal/ molzs), suggesting that an intramolecular strong hydrogen bond forms in the protonated ethoxylated alcohol. A comparison of the entropy of protonation of the ethoxylated alcohol with that of some diols is given in Table 1.13 The comparison suggests that the entropy of the protonated ion of the ethoxylated alcohol is consistent with closure of a (28) Cunningham, A. J.; Payzant,J. D.;Kebarle, P.J. Am.Chem. SOC. 1972,94,1621.
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Figure 10. Van't Hoff plot of the log of equilibrium constants for the reactlon of ((CH3)?NCeH5)H+and C12H2s(OC2HlhOH.
Table I. Entropy of Protonation of Internally H-Bonding Species As of Bp ecies protonation (eu)
Present work. AS for proton transfer from protonated NJVdimethylaniline. Reference 13. As for proton transfer fromaliphatic amines.
large ring by formation of an intramolecular strong hydrogen bond.
ACKNOWLEDGMENT This research was supported in part by a grant from the National Science Foundation (CHE-8813619) to M.S.B.M.
RECEIVED for review October 27, 1992. Accepted April 14, 1993.
