Mass Spectroscopic Observation of an Ester Formation Reaction

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4476

J. Phys. Chem. 1996, 100, 4476-4479

Mass Spectroscopic Observation of an Ester Formation Reaction between Benzoic Acid and Alcohol Initiated by Multiphoton Excitation in a Liquid Beam Fumitaka Mafune´ , Jun-ya Kohno, and Tamotsu Kondow* Department of Chemistry, School of Science, The UniVersity of Tokyo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: September 8, 1995; In Final Form: NoVember 29, 1995X

An alcohol solution of benzoic acid, C6H5COOH, was introduced into a vacuum as a continuous liquid flow (liquid beam) and irradiated with a laser beam at a wavelength of 272 nm; the alcohols used were ethanol and propanol. Ions produced by multiphoton excitation in the liquid beam and ejected from it were analyzed by time-of-flight mass spectrometry. The mass spectra of the ions produced from benzoic acid in these different alcohol solutions led us to conclude that a protonated benzoic acid, C6H5C(OH)2+, produced by laser irradiation reacts with an alcohol molecule, ROH, in the solution and that C6H5C(OH)2(ROH)+ and C6H5C(OH)(OR)+ are produced. Some of the product ions are regarded as reaction intermediates of ester formation from benzoic acid and the alcohols.

Introduction

SCHEME 1

As molecules on a liquid surface are not completely surrounded by other molecules,1 they are liberated more readily from the liquid surface when they acquire sufficient kinetic energy to surmount the solvation energy.2,3 This specificity facilitates studies on reactions occurring in the vicinity of the liquid surface; ionic species taking part in the reactions are ejected into a vacuum by Coulombic repulsion forces during the course of the reactions4-6 and are able to be detected mass spectroscopically. To investigate ion-molecule reactions on a liquid surface, we have explored a technique of introducing a continuous liquid flow into a vacuum (liquid beam).7-11 This technique facilitates application of mass spectroscopy to the studies of a solution surface in combination with laser multiphoton ionization (MPI), which allows us to introduce ionic species into the liquid beam efficiently and selectively; resonant MPI enables us to activate a given functional group of a given species in the liquid. The ions ejected into the vacuum are observed by means of timeof-flight (TOF) mass spectrometry. In our previous study, an acetal formation reaction was investigated in an alcohol solution of phenyl ketone, C6H5COR1.4,5 When an alcohol solution of C6H5COR1 is irradiated with 272 nm laser light, it is excited to the T1 state via an Sn state;12-24 phenyl ketone in the T1 state abstracts hydrogen from the alcohol to form the radical C6H5C(OH)R1. This radical is ionized further by absorbing more photons, and protonated phenyl ketone, C6H5C(OH)R1+, is formed; this ionic species is a reaction intermediate of the acetal formation reaction. Thus, the reaction intermediate is selectively produced by resonant MPI and ejected into the vacuum. In this paper, we applied this technique to investigate an ester formation reaction in an alcohol solution of benzoic acid.25 In the conventional ester formation reaction of a carboxylic acid with an alcohol, a solution of the carboxylic acid in the alcohol is subjected to acidification so as to prepare RC(OH)2+ as a reaction precursor for ester formation. This precursor ion reacts readily with the alcohol to produce RC(OH)2(ROH)+ followed by its dehydration to produce RC(OH)(RO)+ (see Scheme 1). The resonant MPI of benzoic acid also gives C6H5C(OH)2+.4,5 Therefore, the esterification reaction between X

Abstract published in AdVance ACS Abstracts, February 15, 1996.

0022-3654/96/20100-4476$12.00/0

C6H5C(OH)2+ and the alcohol is expected to be initiated by photoionization. The multiphoton chemical reaction was studied, along with the formation mechanism of C6H5C(OH)2+; The formation mechanism of C6H5C(OH)2+ was studied by addition of naphthalene to quench the triplet state of benzoic acid. If triplet benzoic acid is involved in the reaction, the intensities of the ionic species related to the reaction are significantly reduced by introduction of naphthalene in the solution. In addition, cinnamic acid, C6H5C2H2COOH, was tested as one of the typical derivatives of benzoic acid. Experimental Section A continuous laminar liquid flow (liquid beam) of an alcohol solution was introduced into a vacuum chamber from a nozzle having an aperture with a diameter of 20 µm. The flow rate was maintained at 0.2 mL/min with a stagnation pressure of typically 30 atm inside the nozzle; the flow rate was optimized by considering the viscosity of a given solution, the diameter of the nozzle aperture, and the velocity of the liquid flow so that the Reynolds number of the flow was within the suitable range. The liquid beam was trapped 5 cm downstream from the nozzle by a cylindrical cryopump cooled by liquid N2. The liquid beam chamber was evacuated by a 1200 L s-1 diffusion pump. The ambient pressure was typically 10-5-10-6 Torr during injection of the liquid beam. Benzoic acid, cinnamic acid, and naphthalene (Wako Pure Chemical Industries. Ltd. 99% purity) are commercially available and were used without further purification. The liquid beam was crossed with a UV laser beam 3 mm from the nozzle in the first acceleration region of the TOF mass spectrometer. The UV laser beam was obtained by frequency-doubling of the output of a Quanta-ray PDL-3 dye laser pumped by the third harmonic of a Quantaray GCR-3 Nd:YAG laser. The laser power (∼40 µJ/pulse) © 1996 American Chemical Society

MS Observation of Ester Formation

J. Phys. Chem., Vol. 100, No. 11, 1996 4477

Figure 1. TOF mass spectrum of ions produced from a 0.5 M solution of benzoic acid in ethanol by irradiation with a 272 nm laser.

was monitored by a photodiode. The laser was focused into the liquid beam by a lens with a focal length of 450 mm. The mass-to-charge ratio, m/z, of each ion produced by laser photoionization was analyzed by the reflectron TOF mass spectrometer. The ions ejected from the liquid beam were accelerated by a pulsed electric field in the first acceleration region in the direction perpendicular to both the liquid and the laser beams. A delay time from the ionization to the ion extraction was varied in the range 0-1.6 µs to optimize mass resolution.8 The ions were then steered and focused by a set of vertical and horizontal deflectors and an einzel lens. The first and the third plates of the einzel lens were grounded to prepare a field-free region of 1.5 m beyond the einzel lens. The reflectron provided a reversing field tilted by 2° off the beam axis. After traveling a 0.5 m field-free region, the ions were detected by a Murata EMS-6081B Ceratron electron multiplier. Signals from the multiplier were amplified and processed by a Yokogawa DL 1200E transient digitizer based on an NEC 9801 microcomputer. The mass resolution, defined as m/∆m, was 300 under the present experimental conditions.

Figure 2. TOF mass spectra of ions produced from 0.5 M solutions of benzoic acid in ethanol and in n-propanol. The spectra show peaks, which are assignable to C6H5C(OH)2+ (solid circle), C6H5C(OH)2(ROH)+ (open triangle), and C6H5C(OH)(RO)+ (open circle). These dominant peaks shift by m/z ) 14 with an increase in the chain length of the alkyl group of the alcohol.

Results Figure 1 shows a typical TOF mass spectrum of ions produced from a 0.5 M solution of benzoic acid, C6H5COOH, in ethanol by irradiation with a 272 nm laser. Some of the dominant peaks are classified into series of cluster ions: (1) the ion with m/z ) 123, (2) the ion with m/z ) 169 and its solvated cluster ions, (3) the ion with m/z ) 151 and its solvated cluster ions, and (4) the protonated ethanol cluster ions. On the other hand, peaks associated with C6H5COOH+(C2H5OH)n are not observed in the mass spectrum. To identify these peaks, ethanol was replaced with n-propanol, and the m/z values of the product ions were measured. As shown in Figure 2, the ion peaks associated with the ions (2 and 3) shift by m/z ) 14 with the replacement of the solvent from ethanol to n-propanol, whereas the ion (1) does not. This shift indicates that the ionic species associated with the ions (2 and 3) include the alkyl group of the alcohol. By analogy to the results for alcohol solutions of phenyl ketones,4,5 the ions (1-3) are assignable to C6H5C(OH)2+, C6H5C(OH)2(C2H5OH)+(C2H5OH)n, and C6H5C(OH)(C2H5O)+(C2H5OH)n, respectively. Another isomer of C6H5C(OH)2(C2H5OH)+ having the form of C6H5C(OH)(OH2)(C2H5O)+ is likely to be present. The product ions will be identified by means of photodissociation spectroscopy and collision-induced dissociation. Figure 3 shows typical TOF mass spectra of ions produced from a solution of 0.5 M benzoic acid in ethanol (panel a) and ions produced from the mixture of 0.5 M benzoic acid and 0.05

Figure 3. TOF mass spectra of ions produced from 0.5 M solutions of benzoic acid in ethanol (spectrum a) and ions produced from the mixture of 0.5 M benzoic acid and 0.05 M naphthalene in ethanol (spectrum b). Spectrum c shows the spectrum obtained by subtraction of spectrum a from spectrum b.

M naphthalene in ethanol (panel b), together with the difference between panels a and b (panel c). In panel c, the ions whose abundances increase and decrease by adding naphthalene give the positive and the negative peaks, respectively; the intensities of C6H5C(OH)2+, C6H5C(OH)2(C2H5OH)+(C2H5OH)n, and C6H5C(OH)(C2H5O)+(C2H5OH)n decrease, whereas those of C10H8+(C2H5OH)n and H+(C2H5OH)n increase by the introduction of naphthalene. Our preliminary research on the naphthalene solution in ethanol indicates that C10H8+(C2H5OH)n as well as H+(C2H5OH)n is efficiently produced by resonant MPI. Evidently, naphthalene suppresses the formation of C6H5C(OH)2+. Figure 4 shows the abundance of C6H5C(OH)2+ as a function of the irradiation laser power. The ion abundance begins to rise at about 10 µJ/pulse and continues to increase with an increase in the laser power. The number of photons involved in the excitation and the photoionization process was estimated to be 2.9 ( 0.1 according to a simple ion ejection model;9 Three photons are involved in the formation of C6H5C(OH)2+. Two other dominant ions C6H5C(OH)2(C2H5O)+ and C6H5C(OH)(C2H5O)+ show similar laser power dependences.

4478 J. Phys. Chem., Vol. 100, No. 11, 1996

Mafune´ et al. SCHEME 2

Figure 4. The abundance of C6H5C(OH)2+ plotted as a function of the laser power. Solid lines in this figure are obtained from the calculation based on the simple ion ejection model.

Figure 5. TOF mass spectrum of ions produced from 0.5 M solutions of cinnamic acid in ethanol by irradiation with a 272 nm laser. A series of ions solvated by ethanol molecules, (1) C6H5C2H2C(OH)2+, (2) C6H5C2H2C(OH)2C2H5OH+, and (3) C6H5C2H2C(OH)C2H5O+ are observed, together with the protonated dimer ion, (4) H+(C6H5C2H2COOH)2.

Figure 5 shows a typical TOF mass spectrum of ions produced from a 0.5 M solution of cinnamic acid, C6H5C2H2COOH, in ethanol by irradiation with a 272 nm laser. In this spectrum, C6H5C2H2C(OH)2+, C6H5C2H2C(OH)2(C2H5OH)+(C2H5OH)n, and C6H5C2H2C(OH)(C2H5O)+(C2H5OH)n are observed in addition to the protonated dimer cation, H+(C6H5C2H2COOH)2, whereas H+(C2H5OH)n is not observed. Discussion Product Ions. When an aromatic molecule in an alcohol solution is ionized by irradiation of a UV laser, cluster ions which consist of the solute ion and the solvent molecules are typically ejected into the vacuum.7-10 Similar cluster ions are expected to be ejected into the vacuum following multiphoton ionization in the present experiment, but the peaks observed are not explained in terms of any simple combinations of the solute and the solvent molecules. This finding indicates that ion-molecule reactions occur between the solute ion and the solvent molecules before ejection into the vacuum.4-6 As shown in the Results section, the dominant ionic species 2 and 3 contain the alkyl group of the solvent alcohol molecule (see Figure 2). By taking this into consideration, one concludes that the dominant ions observed are assignable to a protonated ion, C6H5C(OH)2+, an intermediate ion, C6H5C(OH)2(C2H5OH)+, a dehydrated ion, C6H5C(OH)(C2H5O)+, and their solvated ions by the ethanol molecules.

Formation Process. Under irradiation of the 272 nm laser, benzoic acid in the ethanol solution is excited to the S1 state and is relaxed very rapidly into the T1 state, instead of further excitation into the ionization continuum, because of very fast intersystem crossing. This scheme is supported by the result that the abundance of the dominant ionic species decreases after introduction of 0.05 M naphthalene into the solution (see Figure 3), since the abundance of the T1 state is greatly reduced by rapid quenching of the T1 state due to energy transfer to naphthalene before it further absorbs photons. Benzoic acid in the T1 state abstracts hydrogen from the alcohol, and C6H5C(OH)2 is formed. When the ketyl radical, C6H5C(OH)2, is ionized by absorbing two more photons, a protonated phenyl ketone, C6H5C(OH)2+, is produced in the ethanol solution. The present result actually shows that three photons are involved in the formation of C6H5C(OH)2+. At a higher laser power of about 50 µJ/pulse, the average lifetime of the S1 state against excitation to the ionization continuum reaches as long as 40 ps. As this lifetime is much longer than that of the S1 state for the decay to the T1 state (lifetime 0.8 ( 0.1 ps),26 almost all of the excited molecules are expected to relax into the T1 state. In fact, the laser power dependences support this relaxation scheme. It is also probable that benzoic acid in the T1 state is directly ionized by absorbing two more photons and the ion thus produced abstracts hydrogen to form C6H5C(OH)2+. In the protonated phenyl ketone, the carbon atom bonded to the OH group is positively charged and is readily attacked by the oxygen atom of an alcohol molecule as a nucleophile, and the ion-molecule complex, C6H5C(OH)2(C2H5OH)+, is formed. This ion is dehydrated to C6H5C(OH)(C2H5O)+ (Scheme 2). The fact that the same number of photons is involved for the formation of C6H5C(OH)2+, C6H5C(OH)2(C2H5OH)+, and C6H5C(OH)(C2H5O)+ supports this reaction scheme: C6H5C(OH)2(C2H5OH)+ and C6H5C(OH)(C2H5O)+ are produced from C6H5C(OH)2+ without the aid of photons. The comparison of the present result with the conventional ester formation reaction (see Scheme 1) leads us to conclude that C6H5C(OH)2+ prepared by laser irradiation reacts with ROH, and C6H5C(OH)2(ROH)+ and C6H5C(OH)(RO)+ are produced. In the present experiment, the precursor, the intermediate, and the product ions of the ester formation reaction are observed in the mass spectrum of the ions ejected from the liquid beam surface irradiated by the 272 nm laser. This ester formation reaction is known to proceed in the liquid phase, but no corresponding reaction in the gas phase has been observed as far as we know. The ion, C6H5C(OH)2+, produced in solution could be liberated from the solution surface into the gas phase and react there with an ethanol molecule to form C6H5C(OH)(C2H5O)+. According to a previous study, however,

MS Observation of Ester Formation the density of the ethanol molecule is not sufficiently high outside the liquid beam that the rate of the ion-molecule reaction is not sizable.4 It is more likely that C6H5C(OH)2+ reacts with an ethanol molecule more efficiently in solution, since the density of the ethanol molecule in the liquid is much higher. It is concluded, therefore, that the ions produced result mainly from the ion-molecule reaction in the liquid. As shown in Figure 1, the protonated ethanol cluster ions are ejected from the liquid beam but are not directly connected to the ester formation reaction. In fact, no protonated ethanol clusters are ejected from the liquid beam of a solution of cinnamic acid in ethanol, although the ester formation reaction of cinnamic acid with ethanol proceeds. Other Carboxylic Acids. Ions ejected from a solution of cinnamic acid, a derivative of benzoic acid, are assigned as a protonated cinnamic acid ion, C6H5C2H2C(OH)2+, an intermediate ion, C6H5C2H2C(OH)2(C2H5OH)+, and a dehydrated ion, C6H5C2H2C(OH)(C2H5O)+ (see Figure 5). Production of these ions shows that the ester formation reaction involving cinnamic acid and ethanol proceeds on the liquid surface after laser irradiation on it. A similar esterification of 3-phenylpropionic acid in ethanol was observed by laser irradiation on the liquid beam of an ethanol solution of 3-phenylpropionic acid. In addition, a protonated dimer ion with ethanol molecules, H+(C6H5C2H2COOH)2(C2H5OH)n (0 e n e 5) are also observed in the mass spectrum of ions produced from an ethanol solution of cinnamic acid (see Figure 5). The chemical formula of the protonated dimer is considered to be C6H5C2H2C(OH)2(C6H5C2H2COOH)+, because the dehydrated species, C6H5C2H2C(OH)OCOC2H2C6H5+, produced from the protonated dimer is present in the solution as shown in the mass spectrum. In the ester formation reaction, the alcohol is much more nucleophilic than the carboxylic acid, so that the mass peak of C6H5C2H2C(OH)2(C2H5OH)+ should be much more intense in the mass spectrum than that of C6H5C2H2C(OH)2(C6H5C2H2COOH)+. Therefore, a comparable abundance of C6H5C2H2C(OH)2(C2H5OH)+ with C6H5C2H2C(OH)2(C6H5C2H2COOH)+ implies that collision between protonated and bare cinnamic acid molecules occurs much more frequently than that between protonated cinnamic acid and alcohol molecules. This frequent collision could result from a pair formation of cinnamic acid molecules on the liquid surface as is the case of benzophenone in ethanol5 and phenol in water.11 Conclusion The present study based on liquid beam multiphoton ionization mass spectroscopy shows that the reaction intermediate, C6H5C(OH)2+, for the ester formation reaction of benzoic acid with ethanol is prepared by multiphoton excitation of benzoic

J. Phys. Chem., Vol. 100, No. 11, 1996 4479 acid in ethanol. Thus, the liquid beam multiphoton excitation allows us to initiate an ion-molecule photochemical reaction through selective ionization of a given functional group of a molecule in solution. Furthermore, the ionic species appearing during the course of the reaction are detected with high sensitivity, and thereby the reaction mechanism can be elucidated on a molecular level. Acknowledgment. The authors are grateful to Dr. Hidehiro Sakurai and Mr. Yu-ichiro Hashimoto for helpful discussions on the reaction mechanism. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. References and Notes (1) Eisenthal, K. B. Acc. Chem. Res. 1993, 26, 636. (2) Faubel, M.; Schlemmer, S.; Toennies, J. P. Z. Phys. D. 1988, 10, 269. (3) Faubel, M.; Kisters, Th. Nature 1989, 339, 527. (4) Kohno, J.; Mafune´, F.; Kondow, T. J. Am. Chem. Soc. 1994, 116, 9801. (5) Kohno, J.; Horimoto, N.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1995, 99, 15627. (6) Matsumura, H.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1995, 99, 5861. (7) Mafune´, F.; Takeda, Y.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1992, 199, 615. (8) Mafune´, F.; Kohno, J.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 7. (9) Mafune´, F.; Kohno, J.; Kondow, T. J. Chin. Chem. Soc. 1995, 42, 449. (10) Mafune´, F.; Takeda, Y.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 234. (11) Mafune´, F.; Hashimoto, Y.; Hashimoto, M.; Kondow, T. J. Phys. Chem., in press. (12) Yang, N. C.; McClure, D. S.; Murov, S. L.; Houser, J. J.; Dusenbery, R. J. Am. Chem. Soc. 1967, 89, 5466. (13) Yang, N. C.; Dusenbery, R. L. J. Am. Chem. Soc. 1968, 90, 5900. (14) Cohen, S. G.; Green, B. J. Am. Chem. Soc. 1969, 91, 6824. (15) Lewis, F. D. J. Phys. Chem. 1970, 74, 3332. (16) Cohen, S. G.; Parola, A.; Parsons, G. H., Jr. Chem. ReV. 1973, 73, 141. (17) Anderson, R. W., Jr.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. Chem. Phys. Lett. 1974, 28, 153. (18) Topp, M. R. Chem. Phys. Lett. 1975, 32, 144. (19) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, 1978. (20) Schaefer, C. G.; Peter, K. S. J. Am. Chem. Soc. 1980, 102, 7566. (21) Encinas, M. V.; Scaiano, J. C. J. Am. Chem. Soc. 1981, 103, 6393. (22) Simon, J. D.; Peters, K. S. J. Am. Chem. Soc. 1982, 104, 6543. (23) Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1985, 107, 6452. (24) Devadoss, C.; Fessenden, R. W. J. Phys. Chem. 1990, 94, 4540. (25) Pine, S. H. Organic Chemistry, 5th ed.; McGraw-Hill Int.: New York, 1987. (26) Meijer, G.; de Vries, M. S.; Hunziker, H. E.; Wendt, H. R. J. Phys. Chem. 1990, 94, 4394.

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