- Email: [email protected]
Ammonia–water complexes on the surface of aqueous solutions observed with sum frequency generation
18 December 1998
Chemical Physics Letters 298 Ž1998. 400–404
Ammonia–water complexes on the surface of aqueous solutions observed with sum frequency generation Danielle Simonelli a , Steve Baldelli b, Mary Jane Shultz
a,)
a
b
Department of Chemistry, Tufts UniÕersity, Medford, MA 02155, USA Department of Chemistry, UniÕersity of California, Berkeley, CA 94720, USA Received 2 September 1998; in final form 8 October 1998
Abstract Sum frequency generation ŽSFG. is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch Ž n 1 . at 3312 cmy1 and a weaker deformation mode Ž2 n4 . at 3200 cmy1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region Ž3000–3450 cmy1 . has interference in the presence of ammonia molecules at the interface. In dilute solution, n 1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquidrvapor interface. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The structure of ammonia–water complexes in bulk liquids and gases has been investigated extensively w1–6x. A layer of adsorbed NH 3 at the surface of aqueous ammonia solutions was proposed by Rice in 1928 w7x. Despite numerous spectroscopic studies of ammonia, both in the gas phase and in solution, this surface species has been largely neglected. In this work, sum frequency generation ŽSFG. provides the first, direct spectroscopic evidence of an ammonia–water complex at the surface of aqueous ammonia solutions. The motivation for studying the interaction of ammonia with different surfaces arises from its importance in heterogeneous catalysis and its relevance to various industrial processes. The structure and )
Corresponding author. E-mail: [email protected]
adsorption characteristics of ammonia on transition metal w8–11x and metal oxide w12–14x surfaces have been studied in great detail, primarily with ultra-high vacuum techniques. The configuration of ammonia on these surfaces is well known, and trends for chemisorption and physisorption are observed for a number of surface-specific experiments. Adsorption sites differ according to the substrate; however, ammonia is generally bonded to a surface via the nitrogen atom lone-pair electrons with the C3 molecular axis perpendicular to the surface plane w10x. At high coverages, however, the molecule is commonly observed to tilt with respect to the surface normal w15x. In addition to solid substrate interactions, gas- and aqueous-phase ammonia participates in numerous atmospheric processes Žsee, e.g., w16x., such as aerosol . formation and NO x reduction. Ammonia Žand NHq 4 plays a unique role in atmospheric chemistry as the
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 2 3 8 - X
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
most significant alkaline gas in the atmosphere, readily reacting with sulfuric, hydrochloric, and nitric acids w17,18x. Therefore, it is desirable to ascertain aqueous-phase structural information of ammonia molecules at liquid interfaces to determine chemical reactivity occurring on heterogeneous surfaces. In this Letter, we report the results of an SFG investigation of concentrated and dilute ammonia– water mixtures at the liquidrvapor interface. The major spectral feature is an intense peak assigned to the symmetric N–H stretch, confirming the presence of NH 3 molecules at the interface. Most importantly, the data permit examination of both water and ammonia at the surface because spectral features due to both species are observed.
401
4 cmy1 ; two spectra are averaged for each sample and plotted with 1 s error bars. In addition, spectra are normalized for infrared intensity variations, as measured through the sample cell headspace. To correct for alignment and daily laser fluctuations, spectra are referenced to the free OH peak of pure water. Double-distilled, concentrated ammonia Žammonium hydroxide, 30 wt% NH 3 . was purchased from GFS Chemicals and used without further purification. Dilute ammonia solutions were made with 18 M V cm water ŽNanopure.. All sample solutions were handled under UHP N2 in glass or Teflon containers. The sample cell was immersed in an ice–water bath.
3. Results and discussion 2. Experimental The technique and theoretical analyses of SFG have been described in detail and are available in the literature w19–21x. Briefly, SFG combines a fixed visible beam with a tunable infrared beam at an interface, to generate the sum of these incident frequencies. When the infrared beam energy matches the frequency of an interfacial molecular vibrational mode, a resonant enhancement of the signal results. Scanning the frequency of infrared light consequently enables one to obtain a vibrational spectrum of molecules at the interface or surface. Since SFG is electric dipole forbidden in centrosymmetric media but allowed at interfaces where inversion symmetry is broken, surface species are distinguished from molecules that reside in the bulk. Because of this surface selectivity, SFG is an effective probe of species at surfaces and interfaces, enabling investigation of them on a molecular level. The instrumentation and optical layout employed in the reported work have been described in an earlier paper w22x. Briefly, the system is based on a pulsed nanosecond Nd:YAG laser that pumps a KTP-based OPOrOPA. The mid-infrared beam generated by the OPOrOPA is tunable from 2700 to 4000 cmy1 with energies of 1–5 mJrpulse and a power density of 50–200 mJrcm2 . The 532 nm visible beam is produced by doubling in KTP with a power density of ; 200 mJrcm2 . Reported spectra are an average of 400 shotsrpoint at a resolution of
Fig. 1 is an SFG spectrum of the neat waterrair interface referenced to the free OH peak of water in the 3000–3800 cmy1 region with ssp polarization combination Žs-polarized sum frequency signal, spolarized visible light, p-polarized infrared light.. This spectrum is in good agreement with previous SFG work w23,24x. The intense, narrow peak at 3700 cmy1 is due to the ‘free OH’ of water. Free OH refers to an OH group projecting out of the water surface, free of hydrogen bonding. The broad feature at ; 3400 cmy1 is due to water in an asymmetrically bonded environment. Lastly, the broad peak at 3200 cmy1 is attributed to the symmetric stretch of water in a symmetric environment. The features at
Fig. 1. SFG spectrum of the neat waterrair interface for ssp polarization at 277 K.
402
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
liquidrvapor interface is shown in Fig. 2a. The dominant feature at 3312 cmy1 is assigned to the N–H Ž n 1 . symmetric stretch of ammonia. This peak is red-shifted by ; 20 cmy1 from the fundamental NH 3 infrared gas-phase absorption, which occurs at 3335.9 cmy1 w25x. This effect is better illustrated in Fig. 3, which shows the gas-phase ammonia absorption compared to the SFG spectrum of aqueous ammonia in the region 3200–3400 cmy1 . The smaller, narrow peak in Fig. 2a just above 3200 cmy1 is assigned to the overtone of n4 , the asymmetric angle deformation mode. The free OH due to water at the surface is not detected at this ammonia concentration. In more dilute solutions, as shown in Fig. 2b and c, the free OH peak begins to emerge. In addition, both the symmetric stretch and the weaker deformation overtone decrease in intensity with bulk mole fraction. The n 1 and 2 n4 peaks rise above broad features due to hydrogen-bonded water from 3000 to 3500 cmy1 . The position and shape of the N–H stretch reflect interactions between ammonia and water at the surface. Raman spectra of solid, liquid, and aqueousphase ammonia indicate the contrast between ammonia-self interaction and ammonia–water interactions w26–30x. The Raman spectra of both solid and liquid ammonia are similar in that the n 1 , 2 n4 , and n 3 Žthe N–H asymmetric stretch. are broad and red-shifted by 30–50 cmy1 with respect to gas-phase peaks w27–30x. The red-shift is greater for the solid than the liquid, which is attributed to a decrease in hydro-
Fig. 2. Ža. SFG spectrum of concentrated ammonia. Žb. x NH 3 s 0.14. Žc. x NH 3 s 0.14. Intensity scale modified to enlarge the view of the free OH peak of water at 3700 cmy1 . Polarization: ssp.
3400 and 3200 cmy1 are associated with hydrogenbonded water and are termed the hydrogen-bonded region. Since water is a major component of these mixtures, this spectrum is important for discussion of the ammonia–water complex at the interface. The ssp SFG spectrum in the range 3000–3800 cmy1 for 0.3 x Ž x s ammonia mole fraction. at the
Fig. 3. SFG of concentrated ammonia, 3200–3400 cmy1 with gas-phase infrared absorption.
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
gen bonding w27,30x. The aqueous solution spectrum, w4,25,26x however, is red-shifted by the same amount as observed in the SFG spectrum. In addition, the width is only 15–20 cmy1 , indicating that ammonia–water interactions are weaker than ammonia-self interactions. Weak ammonia–water interactions are consistent with observation of rotational structure in infrared absorption spectra of aqueous ammonia solutions w25x. The shape of n 1 is due to two effects: Ža. interference with the underlying water sum frequency spectrum and Žb. the rotational structure of the Q branch. As shown in Fig. 1, the hydrogen-bonded region of water is broad with notable intensity from 3300 to 3380 cmy1 . However, with ammonia in the solution, the SFG intensity is near zero in this range. This interference between ammonia and water indicates that the dipoles are opposing each other. In studies of charged surfactants, Gragson and co-workers reported the reordering of interfacial water molecules evidenced by similar interference effects w31x. In addition to interference effects, the shape is very similar to the Raman spectrum of aqueous ammonia w26x. The gas-phase Raman n 1 peak is also unsymmetrical. This is due to the rotational structure of the Q band, further evidence for the weak interaction between ammonia and water. The ammonia–water complex has been the subject of previous investigations. Bulk solution X-ray diffraction studies propose a structural model for the complex similar to that of liquid water w6x. More specifically, ab initio calculations w32x and matrix isolation studies w2,3x conclude that the ammonia– water complex is a hydrogen-bonded structure in which water forms an essentially linear hydrogen bond to ammonia via the nitrogen lone-pair electrons. The ammonia–water surface complex was proposed by Rice based on surface tension determinations w7x. According to these experiments, less than a monolayer of NH 3Žaq. molecules reside on the surface. More recently, calculation of the ammonia mass accommodation coefficient on water has also led to speculation of dissolved NH 3Žaq. close to the liquid surface w33x. This SFG work spectroscopically verifies an ammonia–water surface complex. In concentrated ammonia, the ammonia symmetric stretch peak is remarkably intense in comparison
403
to SFG signals generated by the free OH of neat water, to which the reported spectra are referenced. The observed intensity is due to both the orientation and the Raman and infrared transition strengths. The Raman cross-section for the symmetric stretch of ammonia is double that of water w34x, which is reflected in the larger signal intensity of ammonia. The sharp symmetric stretch peak is a consequence of the narrow orientation distribution of ammonia on the surface of aqueous solutions, as compared to water molecules at the neat waterrvapor interface. Since the spectra do not show shifts in peak position, broadening or new spectral features, the NH 3 orientation remains the same within the concentration range studied Ž0.3 x–0.06 x .. The ppp and pss spectra of 0.3 x NH 3 Žnot shown. did not reveal any characteristic spectral features. The sps spectrum was similar to that of the sps spectrum of neat water, revealing one broad feature at 3400 cmy1 attributed to hydrogen-bonded water. Thus, ssp is the only polarization combination that generates an SFG signal from the surface of aqueous ammonia. Under ssp polarization conditions, the observed signal intensity is due to components of the vibrational transition moments that are normal to the surface. The absence of signal from other polarization combinations suggests that the dipole of ammonia is perpendicular to the surface. Further studies of more dilute solutions are in progress.
4. Conclusions The liquidrvapor interface of aqueous ammonia solutions has been probed using sum frequency generation ŽSFG. and confirms the existence of surface ammonia. The SFG spectra of these mixtures give rise to a sharp N–H symmetric stretch peak at 3312 cmy1 and a less intense peak at ; 3200 cmy1 , which has been assigned to the asymmetric deformation mode. Both are red-shifted from the gas-phase infrared absorption peak positions. The position and shape of the symmetric stretch can be attributed to interference effects and rotational structure. In addition, the SFG spectra are comparable to bulk Raman experiments, in which peak characteristics are ascribed to the extent of hydrogen bonding. Thus, the surface of aqueous ammonium hydroxide consists of
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
404
a network of NH 3Žaq. that are weakly hydrogenbonded to surface water molecules. These observations are evidence for ammonia–water complexes on the surface of aqueous mixtures. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x
S. Suzer, L. Andrews, J. Chem. Phys. 87 Ž1987. 5131. A. Engdahl, B.J. Nelander, Chem. Phys. 91 Ž1989. 6604. B.J. Nelander, L.J. Nord, Phys. Chem. 86 Ž1982. 4375. A. Langseth, Z. Phys. 77 Ž1932. 60. W.B. Fischer, H.-H. Eysel, J. Mol. Struct. 415 Ž1997. 249. A.H. Narten, J. Chem. Phys. 49 Ž1968. 1692. O.K. Rice, J. Phys. Chem. 32 Ž1928. 583. B.A. Sexton, G.E. Mitchell, Surf. Sci. 99 Ž1980. 532. C. Benndorf, T.E. Madey, Surf. Sci. 135 Ž1983. 164. S.T. Ceyer, J.T. Yates Jr., Surf. Sci. 155 Ž1985. 584. A. Chattopadhyay, H. Yang, J.L. Whitten, J. Phys. Chem. 94 Ž1990. 6379. M.-C. Wu, C.M. Truong, D.W. Goodman, J. Phys. Chem. 97 Ž1993. 4182. V. Panella, J. Suzanne, J.-P. Coulomb, Surf. Sci. 350 Ž1996. L211. J. Amores, V. Escribano, G. Ramis, G. Busca, Appl. Catal. B 13 Ž1997. 45. D. Mocuta, J. Ahner, J.T. Yates Jr., Surf. Sci. 383 Ž1997. 299. P.K. Quinn, W.E. Asher, R.J. Charlson, J. Atmos. Chem. 14 Ž1992. 11. F.J. Dentener, P.J. Crutzen, J. Atmos. Chem. 19 Ž1994. 331.
w18x B. Finlayson-Pitts, J. Pitts Jr., Atmospheric Chemistry: Fundamentals and Experimental Techniques, Wiley, New York, 1986. w19x C.D. Bain, J. Chem. Soc., Faraday Trans. 91 Ž1995. 1281. w20x J.H. Hunt, P. Guyot-Sionnest, Y.R. Shen, Chem. Phys. Lett. 133 Ž1987. 189. w21x C. Hirose, N. Akamatsu, K. Domen, Appl. Spectrosc. 46 Ž1992. 1051. w22x S. Baldelli, C. Schnitzer, D.J. Campbell, M.J. Shultz, J. Phys. Chem. B 101 Ž1997. 4607. w23x S. Baldelli, C. Schnitzer, M.J. Shultz, J. Phys. Chem. B 101 Ž1997. 10435. w24x Q. Du, R. Superfine, E. Freysz, Y.R. Shen, Phys. Rev. Lett. 70 Ž1993. 2313. w25x G. Herzberg, Molecular Spectra and Molecular Structure, Van Nostrand Reinhold, New York, 1945. w26x W.B. Fischer, H.-H. Eysel, J. Mol. Struct. 415 Ž1997. 249. w27x C.A. Plint, R.M.B. Small, H.L. Welsh, Can. J. Chem. 32 Ž1954. 653. w28x O.S. Binbrek, A. Anderson, Chem. Phys. Lett. 15 Ž1972. 421. w29x J.W. Lundeen, W.H.J. Koehler, Phys. Chem. 79 Ž1975. 2957. w30x J.J. Lagowski, G.A. Moczygemba, in: J.J. Lagowski ŽEd.., The Chemistry of Non-Aqueous Solvents, II Acidic and Basic Solvents, Academic Press, New York, 1967. w31x D.E. Gragson, B.M. McCarty, G.L. Richmond, J. Am. Chem. Soc. 119 Ž1997. 6144. w32x J.E. Del Bene, J. Phys. Chem. 92 Ž1988. 2874. w33x Q. Shi, P. Davidovits, J.T. Jayne, D.R. Worsnop, C.E. Kolb Žin preparation.. w34x A. Weber, Raman Spectroscopy of Gases and Liquids, Springer, New York, 1979.
Chemical Physics Letters 298 Ž1998. 400–404
Ammonia–water complexes on the surface of aqueous solutions observed with sum frequency generation Danielle Simonelli a , Steve Baldelli b, Mary Jane Shultz
a,)
a
b
Department of Chemistry, Tufts UniÕersity, Medford, MA 02155, USA Department of Chemistry, UniÕersity of California, Berkeley, CA 94720, USA Received 2 September 1998; in final form 8 October 1998
Abstract Sum frequency generation ŽSFG. is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch Ž n 1 . at 3312 cmy1 and a weaker deformation mode Ž2 n4 . at 3200 cmy1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region Ž3000–3450 cmy1 . has interference in the presence of ammonia molecules at the interface. In dilute solution, n 1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquidrvapor interface. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction The structure of ammonia–water complexes in bulk liquids and gases has been investigated extensively w1–6x. A layer of adsorbed NH 3 at the surface of aqueous ammonia solutions was proposed by Rice in 1928 w7x. Despite numerous spectroscopic studies of ammonia, both in the gas phase and in solution, this surface species has been largely neglected. In this work, sum frequency generation ŽSFG. provides the first, direct spectroscopic evidence of an ammonia–water complex at the surface of aqueous ammonia solutions. The motivation for studying the interaction of ammonia with different surfaces arises from its importance in heterogeneous catalysis and its relevance to various industrial processes. The structure and )
Corresponding author. E-mail: [email protected]
adsorption characteristics of ammonia on transition metal w8–11x and metal oxide w12–14x surfaces have been studied in great detail, primarily with ultra-high vacuum techniques. The configuration of ammonia on these surfaces is well known, and trends for chemisorption and physisorption are observed for a number of surface-specific experiments. Adsorption sites differ according to the substrate; however, ammonia is generally bonded to a surface via the nitrogen atom lone-pair electrons with the C3 molecular axis perpendicular to the surface plane w10x. At high coverages, however, the molecule is commonly observed to tilt with respect to the surface normal w15x. In addition to solid substrate interactions, gas- and aqueous-phase ammonia participates in numerous atmospheric processes Žsee, e.g., w16x., such as aerosol . formation and NO x reduction. Ammonia Žand NHq 4 plays a unique role in atmospheric chemistry as the
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 2 3 8 - X
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
most significant alkaline gas in the atmosphere, readily reacting with sulfuric, hydrochloric, and nitric acids w17,18x. Therefore, it is desirable to ascertain aqueous-phase structural information of ammonia molecules at liquid interfaces to determine chemical reactivity occurring on heterogeneous surfaces. In this Letter, we report the results of an SFG investigation of concentrated and dilute ammonia– water mixtures at the liquidrvapor interface. The major spectral feature is an intense peak assigned to the symmetric N–H stretch, confirming the presence of NH 3 molecules at the interface. Most importantly, the data permit examination of both water and ammonia at the surface because spectral features due to both species are observed.
401
4 cmy1 ; two spectra are averaged for each sample and plotted with 1 s error bars. In addition, spectra are normalized for infrared intensity variations, as measured through the sample cell headspace. To correct for alignment and daily laser fluctuations, spectra are referenced to the free OH peak of pure water. Double-distilled, concentrated ammonia Žammonium hydroxide, 30 wt% NH 3 . was purchased from GFS Chemicals and used without further purification. Dilute ammonia solutions were made with 18 M V cm water ŽNanopure.. All sample solutions were handled under UHP N2 in glass or Teflon containers. The sample cell was immersed in an ice–water bath.
3. Results and discussion 2. Experimental The technique and theoretical analyses of SFG have been described in detail and are available in the literature w19–21x. Briefly, SFG combines a fixed visible beam with a tunable infrared beam at an interface, to generate the sum of these incident frequencies. When the infrared beam energy matches the frequency of an interfacial molecular vibrational mode, a resonant enhancement of the signal results. Scanning the frequency of infrared light consequently enables one to obtain a vibrational spectrum of molecules at the interface or surface. Since SFG is electric dipole forbidden in centrosymmetric media but allowed at interfaces where inversion symmetry is broken, surface species are distinguished from molecules that reside in the bulk. Because of this surface selectivity, SFG is an effective probe of species at surfaces and interfaces, enabling investigation of them on a molecular level. The instrumentation and optical layout employed in the reported work have been described in an earlier paper w22x. Briefly, the system is based on a pulsed nanosecond Nd:YAG laser that pumps a KTP-based OPOrOPA. The mid-infrared beam generated by the OPOrOPA is tunable from 2700 to 4000 cmy1 with energies of 1–5 mJrpulse and a power density of 50–200 mJrcm2 . The 532 nm visible beam is produced by doubling in KTP with a power density of ; 200 mJrcm2 . Reported spectra are an average of 400 shotsrpoint at a resolution of
Fig. 1 is an SFG spectrum of the neat waterrair interface referenced to the free OH peak of water in the 3000–3800 cmy1 region with ssp polarization combination Žs-polarized sum frequency signal, spolarized visible light, p-polarized infrared light.. This spectrum is in good agreement with previous SFG work w23,24x. The intense, narrow peak at 3700 cmy1 is due to the ‘free OH’ of water. Free OH refers to an OH group projecting out of the water surface, free of hydrogen bonding. The broad feature at ; 3400 cmy1 is due to water in an asymmetrically bonded environment. Lastly, the broad peak at 3200 cmy1 is attributed to the symmetric stretch of water in a symmetric environment. The features at
Fig. 1. SFG spectrum of the neat waterrair interface for ssp polarization at 277 K.
402
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
liquidrvapor interface is shown in Fig. 2a. The dominant feature at 3312 cmy1 is assigned to the N–H Ž n 1 . symmetric stretch of ammonia. This peak is red-shifted by ; 20 cmy1 from the fundamental NH 3 infrared gas-phase absorption, which occurs at 3335.9 cmy1 w25x. This effect is better illustrated in Fig. 3, which shows the gas-phase ammonia absorption compared to the SFG spectrum of aqueous ammonia in the region 3200–3400 cmy1 . The smaller, narrow peak in Fig. 2a just above 3200 cmy1 is assigned to the overtone of n4 , the asymmetric angle deformation mode. The free OH due to water at the surface is not detected at this ammonia concentration. In more dilute solutions, as shown in Fig. 2b and c, the free OH peak begins to emerge. In addition, both the symmetric stretch and the weaker deformation overtone decrease in intensity with bulk mole fraction. The n 1 and 2 n4 peaks rise above broad features due to hydrogen-bonded water from 3000 to 3500 cmy1 . The position and shape of the N–H stretch reflect interactions between ammonia and water at the surface. Raman spectra of solid, liquid, and aqueousphase ammonia indicate the contrast between ammonia-self interaction and ammonia–water interactions w26–30x. The Raman spectra of both solid and liquid ammonia are similar in that the n 1 , 2 n4 , and n 3 Žthe N–H asymmetric stretch. are broad and red-shifted by 30–50 cmy1 with respect to gas-phase peaks w27–30x. The red-shift is greater for the solid than the liquid, which is attributed to a decrease in hydro-
Fig. 2. Ža. SFG spectrum of concentrated ammonia. Žb. x NH 3 s 0.14. Žc. x NH 3 s 0.14. Intensity scale modified to enlarge the view of the free OH peak of water at 3700 cmy1 . Polarization: ssp.
3400 and 3200 cmy1 are associated with hydrogenbonded water and are termed the hydrogen-bonded region. Since water is a major component of these mixtures, this spectrum is important for discussion of the ammonia–water complex at the interface. The ssp SFG spectrum in the range 3000–3800 cmy1 for 0.3 x Ž x s ammonia mole fraction. at the
Fig. 3. SFG of concentrated ammonia, 3200–3400 cmy1 with gas-phase infrared absorption.
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
gen bonding w27,30x. The aqueous solution spectrum, w4,25,26x however, is red-shifted by the same amount as observed in the SFG spectrum. In addition, the width is only 15–20 cmy1 , indicating that ammonia–water interactions are weaker than ammonia-self interactions. Weak ammonia–water interactions are consistent with observation of rotational structure in infrared absorption spectra of aqueous ammonia solutions w25x. The shape of n 1 is due to two effects: Ža. interference with the underlying water sum frequency spectrum and Žb. the rotational structure of the Q branch. As shown in Fig. 1, the hydrogen-bonded region of water is broad with notable intensity from 3300 to 3380 cmy1 . However, with ammonia in the solution, the SFG intensity is near zero in this range. This interference between ammonia and water indicates that the dipoles are opposing each other. In studies of charged surfactants, Gragson and co-workers reported the reordering of interfacial water molecules evidenced by similar interference effects w31x. In addition to interference effects, the shape is very similar to the Raman spectrum of aqueous ammonia w26x. The gas-phase Raman n 1 peak is also unsymmetrical. This is due to the rotational structure of the Q band, further evidence for the weak interaction between ammonia and water. The ammonia–water complex has been the subject of previous investigations. Bulk solution X-ray diffraction studies propose a structural model for the complex similar to that of liquid water w6x. More specifically, ab initio calculations w32x and matrix isolation studies w2,3x conclude that the ammonia– water complex is a hydrogen-bonded structure in which water forms an essentially linear hydrogen bond to ammonia via the nitrogen lone-pair electrons. The ammonia–water surface complex was proposed by Rice based on surface tension determinations w7x. According to these experiments, less than a monolayer of NH 3Žaq. molecules reside on the surface. More recently, calculation of the ammonia mass accommodation coefficient on water has also led to speculation of dissolved NH 3Žaq. close to the liquid surface w33x. This SFG work spectroscopically verifies an ammonia–water surface complex. In concentrated ammonia, the ammonia symmetric stretch peak is remarkably intense in comparison
403
to SFG signals generated by the free OH of neat water, to which the reported spectra are referenced. The observed intensity is due to both the orientation and the Raman and infrared transition strengths. The Raman cross-section for the symmetric stretch of ammonia is double that of water w34x, which is reflected in the larger signal intensity of ammonia. The sharp symmetric stretch peak is a consequence of the narrow orientation distribution of ammonia on the surface of aqueous solutions, as compared to water molecules at the neat waterrvapor interface. Since the spectra do not show shifts in peak position, broadening or new spectral features, the NH 3 orientation remains the same within the concentration range studied Ž0.3 x–0.06 x .. The ppp and pss spectra of 0.3 x NH 3 Žnot shown. did not reveal any characteristic spectral features. The sps spectrum was similar to that of the sps spectrum of neat water, revealing one broad feature at 3400 cmy1 attributed to hydrogen-bonded water. Thus, ssp is the only polarization combination that generates an SFG signal from the surface of aqueous ammonia. Under ssp polarization conditions, the observed signal intensity is due to components of the vibrational transition moments that are normal to the surface. The absence of signal from other polarization combinations suggests that the dipole of ammonia is perpendicular to the surface. Further studies of more dilute solutions are in progress.
4. Conclusions The liquidrvapor interface of aqueous ammonia solutions has been probed using sum frequency generation ŽSFG. and confirms the existence of surface ammonia. The SFG spectra of these mixtures give rise to a sharp N–H symmetric stretch peak at 3312 cmy1 and a less intense peak at ; 3200 cmy1 , which has been assigned to the asymmetric deformation mode. Both are red-shifted from the gas-phase infrared absorption peak positions. The position and shape of the symmetric stretch can be attributed to interference effects and rotational structure. In addition, the SFG spectra are comparable to bulk Raman experiments, in which peak characteristics are ascribed to the extent of hydrogen bonding. Thus, the surface of aqueous ammonium hydroxide consists of
D. Simonelli et al.r Chemical Physics Letters 298 (1998) 400–404
404
a network of NH 3Žaq. that are weakly hydrogenbonded to surface water molecules. These observations are evidence for ammonia–water complexes on the surface of aqueous mixtures. References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x
S. Suzer, L. Andrews, J. Chem. Phys. 87 Ž1987. 5131. A. Engdahl, B.J. Nelander, Chem. Phys. 91 Ž1989. 6604. B.J. Nelander, L.J. Nord, Phys. Chem. 86 Ž1982. 4375. A. Langseth, Z. Phys. 77 Ž1932. 60. W.B. Fischer, H.-H. Eysel, J. Mol. Struct. 415 Ž1997. 249. A.H. Narten, J. Chem. Phys. 49 Ž1968. 1692. O.K. Rice, J. Phys. Chem. 32 Ž1928. 583. B.A. Sexton, G.E. Mitchell, Surf. Sci. 99 Ž1980. 532. C. Benndorf, T.E. Madey, Surf. Sci. 135 Ž1983. 164. S.T. Ceyer, J.T. Yates Jr., Surf. Sci. 155 Ž1985. 584. A. Chattopadhyay, H. Yang, J.L. Whitten, J. Phys. Chem. 94 Ž1990. 6379. M.-C. Wu, C.M. Truong, D.W. Goodman, J. Phys. Chem. 97 Ž1993. 4182. V. Panella, J. Suzanne, J.-P. Coulomb, Surf. Sci. 350 Ž1996. L211. J. Amores, V. Escribano, G. Ramis, G. Busca, Appl. Catal. B 13 Ž1997. 45. D. Mocuta, J. Ahner, J.T. Yates Jr., Surf. Sci. 383 Ž1997. 299. P.K. Quinn, W.E. Asher, R.J. Charlson, J. Atmos. Chem. 14 Ž1992. 11. F.J. Dentener, P.J. Crutzen, J. Atmos. Chem. 19 Ž1994. 331.
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