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Existence of hydrogen bonding between the hydroxyl radical and hydrogen peroxide: OH·H2O2
13 August 1999
Chemical Physics Letters 309 Ž1999. 274–278 www.elsevier.nlrlocatercplett
Existence of hydrogen bonding between the hydroxyl radical and hydrogen peroxide: OH P H 2 O 2 Baoshan Wang, Hua Hou, Yueshu Gu
)
School of Chemistry, Shandong UniÕersity, Jinan 250 100, PR China Received 30 March 1999; in final form 17 June 1999
Abstract The hydrogen bonding between the OH radical and the H 2 O 2 molecule has been studied using ab initio molecular orbital methods. The OH P H 2 O 2 complex has a five-membered ring like structure with two distorted hydrogen bonds. The vibrational spectrum is reported. The binding energy D 0 of the OH P H 2 O 2 complex is predicted to be ; 4.1 kcalrmol. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Many hydrogen-bonded complexes have received considerable attention w1–9x. Recently, we have initiated a systematic study of the hydrogen bonding between the hydroxyl radicals and the other molecules using high-level ab initio molecular orbital theory w10,11x. In this Letter, we report our theoretical calculations for characterizing a new hydrogen bond: OH P H 2 O 2 . It has been suggested that the OH P H 2 O 2 radical complex is formed in the OH q H 2 O 2 reaction. Troe et al. w12,13x found that this reaction exhibits anomalous temperature coefficients. A strong up-turn of the rate constant occurs near 800 K. This interesting property can be interpreted reasonably in terms of a complex-forming mechanism. However, both highlevel ab initio calculations and experimental trapping
) Corresponding author. Fax: q86 531 8565403; e-mail: [email protected]
of the transient intermediate are desirable in order to draw the detailed conclusion. To date, no study has been reported on the experimental detection of an OH P H 2 O 2 complex, although one of its isomers ŽHO 2 P H 2 O. has been well characterized both experimentally and theoretically w8,9x. In this Letter, we use ab initio molecular orbital methods to search for hydrogen bonding between the OH radical and the H 2 O 2 molecule. The structure, vibrational spectrum, and binding energy of the OH P H 2 O 2 complex are determined. Our theoretical predictions can act as a guide for future experimental observation of the complex.
2. Computational methods All calculations were performed using the Gaussian 94 programs w14x. Geometries of the OH P H 2 O 2 complex were fully optimized without symmetry restrictions. Both unrestricted second-order Møller– Plesset theory wUMP2Žfull.x w15x and the hybrid den-
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 8 6 - 7
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
275
sity functional B3LYP method, i.e., Becke’s threeparameter nonlocal exchange functional w16x with the correlation functional of Lee, Yang, and Parr w17x, with the 6-311 q q GŽd, p. basis set, were used in the calculation. The vibrational spectrum Žfrequencies and infrared intensities. was calculated at the same levels of theory. Single-point energies were obtained using the coupled cluster wCCSDŽT.x method w18–21x with the 6-311q q GŽ2d, 2p. basis set. The basis set superposition errors were estimated using the full Boys-Bernardi counterpoise correction scheme w22x. We have examined the spin contamination before and after annihilation for two open-shell species: OH and the OH P H 2 O 2 complex. Before annihilation, the expectation ² S 2 : values of UHF references for these two species are the same: 0.756 at UMP2 level and 0.752 at UB3LYP level. After annihilation, the values of ² S 2 : are 0.750 Žthe exact value for a pure doublet. at both UMP2 and UB3LYP levels for both species. This indicates not only that the wavefunction is not strongly contaminated by states of higher multiplicity but that a single determinant reference wavefunction for these systems is suitable for the various levels of theory used in the optimization.
3. Results and discussion
Fig. 1. Optimized geometries for the OHPH 2 O 2 complex and the monomers at the UMP2Žfull.r6-311qqGŽd, p. Župper numbers. and the UB3LYPr6-311qqGŽd, p. Žlower numbers. levels. Bond distances are in angstrom ˚ ¨ and the angles are in degree.
3.1. Geometries The fully optimized geometries for the OH P H 2 O 2 complex and the monomers are depicted in Fig. 1. We did explore other possible hydrogen bonds but found that these structures are not genuine minima. It is obvious that both UMP2Žfull. and UB3LYP methods yield similar geometries. Fig. 1 shows that the OH P H 2 O 2 complex involves two hydrogen bonds. One is the hydrogen bond between the hydrogen atom in the OH radical and one of the oxygen atoms in the H 2 O 2 ŽHY PPP OX .. The other hydrogen bond occurs between the oxygen atom in the hydroxyl radical and one of the hydrogen atoms in the hydrogen peroxide ŽOY PPP H.. So, OH P H 2 O 2 has a floppy five-member-ring structure with the other hydrogen atom in the H 2 O 2 out of the plane. At the UMP2Žfull.r6-311q q GŽd, p. level, the HY OX and OY H hydrogen bonds distances are calculated to be
˚ respectively, which are 0.059 2.148 and 2.150 A, ˚ and 0.046 A longer than those obtained at the UB3LYPr6-311q q GŽd, p. level. In the complex, ˚ longer than the OY HY bond distance is ; 0.005 A that of the isolated OH radical. The geometrical parameters of the H 2 O 2 moiety are little different from those of the isolated H 2 O 2 molecule. The O–OX and O–H bonds are elongated by 0.001 and ˚ respectively. 0.005 A, The rotational constants for the complex and the monomers are also reported in Table 1. We also reported the geometric parameters which can be measured experimentally, namely the distances between the hydrogen-bonded heavy atoms, as listed in Table 1. It is evident that the OH P H 2 O 2 complex is an asymmetric rotor. Since this hydrogen bond has a permanent dipole moment which is somewhat larger
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
276
Table 1 ˚ ., angles Žin degree., rotational constants Žin MHz. and the dipole moments Ž m, in debye. for the OH P H 2 O 2 complex and Distances Žin A the monomers calculated at the UMP2Žfull.r6-311q q GŽd, p. and UB3LYPr6-311q q GŽd, p. Žin Italics . levels Species
Distances Y
OH P H 2 O 2
Angles X
Y
m
Rotational constants
Y
Y
Y
X
OO
OO
HOO
H O O
2.955 2.909
2.854 2.816
28.2 28.5
36.2 35.1
BA
BB
2 5803 25 517
5 692 5 882
OH
569 083 559 756
569 083 559 756
H 2 O2
303 037 303 382
26 736 26 471
than those of the monomers, it should be active in the microwave region of the spectrum w9x. 3.2. Energetics The calculated total energies and binding energies for the OH P H 2 O 2 complex are given in Table 2. Whether the MP2 or B3LYP optimized geometries are used in the single-point energy calculations, the MP2, CCSDŽT., and B3LYP methods predict almost the same binding energies for the OH P H 2 O 2 complex. The change in geometry leads to an energy change of only ; 0.1 kcalrmol. This may be caused by the flatness of the potential energy surface for such a weakly bound complex. At the highest level of theory wCCSDŽT.x, the De is calculated to be ; 5.9 kcalrmol for OH P H 2 O 2 . Corrections for zero-point energy yield a value of D 0 of ; 4.1 kcalrmol. The basis set superposition error is estimated to be - 1.0 kcalrmol. With such a large binding energy, the OH P H 2 O 2 complex should be experimentally observable. It is interesting to compare the stability of OH P H 2 O 2 with several analogous hydrogen-bonded com-
BC 4 711 4 832
1.982 1.912 1.908 1.832
25 586 25 355
1.812 1.732
plexes on the basis of the reported binding energies. For example, the binding energies Ž D 0 . have been predicted to be 1.7, 3.6, 3.8, and 6.9 kcalrmol for the most stable conformers of the OH P H 2 S w11x, H 2 O P H 2 O w6x, OH P H 2 O w10x, and HO 2 P H 2 O w9x complexes, respectively. Therefore, the OH P H 2 O 2 complex has a similar dissociation energy to the H 2 O P H 2 O and OH P H 2 O complexes. However, it is more tightly bound than the OH P H 2 S complex by 2.4 kcalrmol and less stable than the HO 2 P H 2 O complex by ; 2.8 kcalrmol. 3.3. Frequencies In Table 3, the harmonic vibrational frequencies and the corresponding infrared intensities for the OH P H 2 O 2 complex are shown. To compare with the vibrational frequencies of the isolated OH and H 2 O 2 , the frequency shifts of the complex relative to OH q H 2 O 2 are also listed. It is worth noting that although the UMP2 and UB3LYP methods yield similar geometries and energies for the OH P H 2 O 2 complex, they predict apparently different vibrational spectra. Inconsistent frequency shifts are also pre-
Table 2 Total energies Žin hartree. and binding energies Ž De and D 0 , in kcal moly1 . for the OH P H 2 O 2 complex Methods
OH P H 2 O 2
OH
H 2 O2
De
D0
MP2rrMP2 CCSDŽT.rrMP2 B3LYPrrB3LYP MP2rrB3LYP CCSDŽT.rrB3LYP
y226.88993 y226.93082 y227.37381 y226.88982 y226.93086
y75.59737 y75.61491 y75.76241 y75.59729 y75.61489
y151.28332 y151.30656 y151.60217 y151.28326 y151.30659
5.80 5.87 5.79 5.82 5.89
4.06 4.13 3.97 4.00 4.07
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
277
Table 3 Harmonic vibrational frequencies Žin cmy1 . and intensities Žin km moly1 . for the OH P H 2 O 2 complex at the UMP2Žfull.r6-311q q GŽd, p. and UB3LYPr6-311q q GŽd, p. levels No.
1 2 3 4 5 6 7 8 9 10 11 12 a b
Mode description
OH asym. stretching OH sym. stretching Y Y O H stretching OH sym. bending OH asym. bending X OO stretching intermolecular X X HOO H torsion intermolecular intermolecular intermolecular intermolecular
Frequenciesa
Intensitiesb
UMP2Žfull.
UB3LYP
UMP2Žfull.
UB3LYP
3 852.2 Žq0.1. 3 781.6 Žy69.5. 3 755.5 Žy87.2. 1 491.4 Žq32.6. 1 319.7 Žq17.9. 922.5 Žq0.1. 517.0 407.5 Žq12.7. 260.8 237.1 165.3 129.5
3 780.2 Žq3.2. 3 650.1 Žy125.9. 3 601.4 Žy105.5. 1 495.7 Žq42.0. 1 316.9 Žq20.4. 931.5 Žy3.1. 559.5 415.2 Žq48.3. 281.7 225.1 182.3 143.5
51.7 Ž3.2. 205.6 Ž2.9. 13.5 Ž0.7. 8.6 109.8 Ž1.1. 1.5 Ž1.4. 267.3 253.4 Ž1.1. 31.5 85.6 3.7 45.2
50.8 Ž3.8. 239.1 Ž3.5. 21.2 Ž1.6. 13.2 98.6 Ž1.1. 1.9 Ž1.7. 250.1 276.6 Ž1.2. 51.6 72.7 2.6 36.0
Numbers in parentheses are frequency shifts relative to the monomers. Numbers in parentheses are ratios of intensities for the complex relative to the monomers.
dicted. Because there are no experimental studies of the OH P H 2 O 2 complex, it is difficulty for us to determine which predictions should be more reliable. However, several important conclusions still can be drawn from the present high-level theoretical calculations. Both MP2 and B3LYP methods predict that there are four strong bands in the infrared spectrum of the OH P H 2 O 2 complex. They involve three intramolecular vibrational modes ŽHOOX HX torsion Ž n 8 ., OH symmetric stretching Ž n 2 ., and OH asymmetric bending Ž n 5 .. and one intermolecular vibrational mode Ž n 7 .. It is interesting to note that all three of these intramolecular modes belong to the H 2 O 2 moiety. To compare with the corresponding vibrational modes of the isolated H 2 O 2 molecule, the intensities of the n 8 and n 5 modes barely change while the intensity of n 2 mode increases by about three times. Both MP2 and B3LYP methods predict that the OY HY stretching mode Ž n 3 . in the OH moiety and the OH symmetric stretching mode Ž n 2 . in the H 2 O 2 moiety are significantly red-shifted. Another three bands in the H 2 O 2 moiety, OH asymmetric stretching Ž n 1 ., OH symmetric stretching Ž n4 ., and OH asymmetric bending Ž n 8 ., are predicted to be blueshifted. The largest blue-shift of 32.6–42.0 cmy1 results in the n4 mode. Moreover, this Raman active mode in the isolated H 2 O 2 molecule becomes weakly
infrared active in the complex with an intensity of 8.6–13.2 kmrmol. There are five intermolecular modes Ž n 7 , n 9 , n 10 , n 11 and n 12 .. For these new intermolecular modes the MP2 and B3LYP methods predict very similar vibrational fundamental frequencies and infrared intensities. Moreover, the n 7 mode is predicted to be one of the most intense bands in the infrared spectrum of the complex. The observation of these vibrational modes in the laboratory would support the existence of the OH P H 2 O 2 radical complex.
4. Conclusions The hydrogen bonding between the OH radical and the H 2 O 2 molecule has been studied using both UMP2Žfull. and UB3LYP methods with the 6-311 q q GŽd, p. basis set. The equilibrium structure of the OH P H 2 O complex is predicted to be a floppy five-membered ring involving two distorted O PPP H PPP O hydrogen bonds with O–O distances ˚ The binding energy D 0 of of ; 2.9 and ; 2.8 A. the OH P H 2 O 2 complex is calculated to be ; 4.1 kcalrmol at the CCSDŽT.r6-311 q q GŽ2d, 2p. level of theory. The infrared spectrum of the complex is also predicted.
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x
A.C. Legon, D.J. Millen, Chem. Rev. 86 Ž1986. 635. A.C. Legon, D.J. Millen, Acc. Chem. Res. 20 Ž1987. 39. L.A. Curtiss, M. Blancker, Chem. Rev. 88 Ž1988. 963. P. Hobza, R. Zahradnik, Chem. Rev. 88 Ž1988. 871. D.A. Smith, Modeling the Hydrogen Bond, Am. Chem. Soc., Washington, DC, 1994. K.S. Kim, H.S. Kim, J.H. Jung, B.J. Mhin, Y. Xie, H.F. Schaefer, J. Chem. Phys. 94 Ž1991. 2057. Y. Xie, H.F. Schaefer, J. Chem. Phys. 98 Ž1993. 8829. B. Nelander, J. Phys. Chem. A 101 Ž1997. 9092. S. Aloisio, J.S. Francisco, J. Phys. Chem. A 102 Ž1998. 1899. B. Wang, H. Hou, Y. Gu, Chem. Phys. Lett. 303 Ž1999. 96. B. Wang, H. Hou, Y. Gu Žsubmitted.. H. Hippler, J. Troe, Chem. Phys. Lett. 192 Ž1992. 333. H. Hippler, H. Neunaber, J. Troe, J. Chem. Phys. 103 Ž1995. 3510.
w14x M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.W.M. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Allaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzales, J.A. Pople, Gaussian 94, Revision E.1, Gaussian, Inc., Pittsburgh, PA, 1995. w15x C. Møller, M.S. Plesset, Phys. Rev. 46 Ž1934. 618. w16x A.D. Becke, J. Chem. Phys. 98 Ž1993. 1372. w17x C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 41 Ž1988. 785. w18x J. Cizek, J. Chem. Phys. 45 Ž1966. 4256. w19x G.D. Purvis, R.J. Bartlett, J. Chem. Phys. 76 Ž1982. 1910. w20x J.A. Pople, M. Head-Gordon, K. Raghavachari, J. Chem. Phys. 87 Ž1987. 5968. w21x T.J. Lee, A.P. Rendell, J. Chem. Phys. 69 Ž1991. 399. w22x S.F. Boys, F. Bernardi, Mol. Phys. 19 Ž1970. 553.
Chemical Physics Letters 309 Ž1999. 274–278 www.elsevier.nlrlocatercplett
Existence of hydrogen bonding between the hydroxyl radical and hydrogen peroxide: OH P H 2 O 2 Baoshan Wang, Hua Hou, Yueshu Gu
)
School of Chemistry, Shandong UniÕersity, Jinan 250 100, PR China Received 30 March 1999; in final form 17 June 1999
Abstract The hydrogen bonding between the OH radical and the H 2 O 2 molecule has been studied using ab initio molecular orbital methods. The OH P H 2 O 2 complex has a five-membered ring like structure with two distorted hydrogen bonds. The vibrational spectrum is reported. The binding energy D 0 of the OH P H 2 O 2 complex is predicted to be ; 4.1 kcalrmol. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Many hydrogen-bonded complexes have received considerable attention w1–9x. Recently, we have initiated a systematic study of the hydrogen bonding between the hydroxyl radicals and the other molecules using high-level ab initio molecular orbital theory w10,11x. In this Letter, we report our theoretical calculations for characterizing a new hydrogen bond: OH P H 2 O 2 . It has been suggested that the OH P H 2 O 2 radical complex is formed in the OH q H 2 O 2 reaction. Troe et al. w12,13x found that this reaction exhibits anomalous temperature coefficients. A strong up-turn of the rate constant occurs near 800 K. This interesting property can be interpreted reasonably in terms of a complex-forming mechanism. However, both highlevel ab initio calculations and experimental trapping
) Corresponding author. Fax: q86 531 8565403; e-mail: [email protected]
of the transient intermediate are desirable in order to draw the detailed conclusion. To date, no study has been reported on the experimental detection of an OH P H 2 O 2 complex, although one of its isomers ŽHO 2 P H 2 O. has been well characterized both experimentally and theoretically w8,9x. In this Letter, we use ab initio molecular orbital methods to search for hydrogen bonding between the OH radical and the H 2 O 2 molecule. The structure, vibrational spectrum, and binding energy of the OH P H 2 O 2 complex are determined. Our theoretical predictions can act as a guide for future experimental observation of the complex.
2. Computational methods All calculations were performed using the Gaussian 94 programs w14x. Geometries of the OH P H 2 O 2 complex were fully optimized without symmetry restrictions. Both unrestricted second-order Møller– Plesset theory wUMP2Žfull.x w15x and the hybrid den-
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 6 8 6 - 7
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
275
sity functional B3LYP method, i.e., Becke’s threeparameter nonlocal exchange functional w16x with the correlation functional of Lee, Yang, and Parr w17x, with the 6-311 q q GŽd, p. basis set, were used in the calculation. The vibrational spectrum Žfrequencies and infrared intensities. was calculated at the same levels of theory. Single-point energies were obtained using the coupled cluster wCCSDŽT.x method w18–21x with the 6-311q q GŽ2d, 2p. basis set. The basis set superposition errors were estimated using the full Boys-Bernardi counterpoise correction scheme w22x. We have examined the spin contamination before and after annihilation for two open-shell species: OH and the OH P H 2 O 2 complex. Before annihilation, the expectation ² S 2 : values of UHF references for these two species are the same: 0.756 at UMP2 level and 0.752 at UB3LYP level. After annihilation, the values of ² S 2 : are 0.750 Žthe exact value for a pure doublet. at both UMP2 and UB3LYP levels for both species. This indicates not only that the wavefunction is not strongly contaminated by states of higher multiplicity but that a single determinant reference wavefunction for these systems is suitable for the various levels of theory used in the optimization.
3. Results and discussion
Fig. 1. Optimized geometries for the OHPH 2 O 2 complex and the monomers at the UMP2Žfull.r6-311qqGŽd, p. Župper numbers. and the UB3LYPr6-311qqGŽd, p. Žlower numbers. levels. Bond distances are in angstrom ˚ ¨ and the angles are in degree.
3.1. Geometries The fully optimized geometries for the OH P H 2 O 2 complex and the monomers are depicted in Fig. 1. We did explore other possible hydrogen bonds but found that these structures are not genuine minima. It is obvious that both UMP2Žfull. and UB3LYP methods yield similar geometries. Fig. 1 shows that the OH P H 2 O 2 complex involves two hydrogen bonds. One is the hydrogen bond between the hydrogen atom in the OH radical and one of the oxygen atoms in the H 2 O 2 ŽHY PPP OX .. The other hydrogen bond occurs between the oxygen atom in the hydroxyl radical and one of the hydrogen atoms in the hydrogen peroxide ŽOY PPP H.. So, OH P H 2 O 2 has a floppy five-member-ring structure with the other hydrogen atom in the H 2 O 2 out of the plane. At the UMP2Žfull.r6-311q q GŽd, p. level, the HY OX and OY H hydrogen bonds distances are calculated to be
˚ respectively, which are 0.059 2.148 and 2.150 A, ˚ and 0.046 A longer than those obtained at the UB3LYPr6-311q q GŽd, p. level. In the complex, ˚ longer than the OY HY bond distance is ; 0.005 A that of the isolated OH radical. The geometrical parameters of the H 2 O 2 moiety are little different from those of the isolated H 2 O 2 molecule. The O–OX and O–H bonds are elongated by 0.001 and ˚ respectively. 0.005 A, The rotational constants for the complex and the monomers are also reported in Table 1. We also reported the geometric parameters which can be measured experimentally, namely the distances between the hydrogen-bonded heavy atoms, as listed in Table 1. It is evident that the OH P H 2 O 2 complex is an asymmetric rotor. Since this hydrogen bond has a permanent dipole moment which is somewhat larger
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
276
Table 1 ˚ ., angles Žin degree., rotational constants Žin MHz. and the dipole moments Ž m, in debye. for the OH P H 2 O 2 complex and Distances Žin A the monomers calculated at the UMP2Žfull.r6-311q q GŽd, p. and UB3LYPr6-311q q GŽd, p. Žin Italics . levels Species
Distances Y
OH P H 2 O 2
Angles X
Y
m
Rotational constants
Y
Y
Y
X
OO
OO
HOO
H O O
2.955 2.909
2.854 2.816
28.2 28.5
36.2 35.1
BA
BB
2 5803 25 517
5 692 5 882
OH
569 083 559 756
569 083 559 756
H 2 O2
303 037 303 382
26 736 26 471
than those of the monomers, it should be active in the microwave region of the spectrum w9x. 3.2. Energetics The calculated total energies and binding energies for the OH P H 2 O 2 complex are given in Table 2. Whether the MP2 or B3LYP optimized geometries are used in the single-point energy calculations, the MP2, CCSDŽT., and B3LYP methods predict almost the same binding energies for the OH P H 2 O 2 complex. The change in geometry leads to an energy change of only ; 0.1 kcalrmol. This may be caused by the flatness of the potential energy surface for such a weakly bound complex. At the highest level of theory wCCSDŽT.x, the De is calculated to be ; 5.9 kcalrmol for OH P H 2 O 2 . Corrections for zero-point energy yield a value of D 0 of ; 4.1 kcalrmol. The basis set superposition error is estimated to be - 1.0 kcalrmol. With such a large binding energy, the OH P H 2 O 2 complex should be experimentally observable. It is interesting to compare the stability of OH P H 2 O 2 with several analogous hydrogen-bonded com-
BC 4 711 4 832
1.982 1.912 1.908 1.832
25 586 25 355
1.812 1.732
plexes on the basis of the reported binding energies. For example, the binding energies Ž D 0 . have been predicted to be 1.7, 3.6, 3.8, and 6.9 kcalrmol for the most stable conformers of the OH P H 2 S w11x, H 2 O P H 2 O w6x, OH P H 2 O w10x, and HO 2 P H 2 O w9x complexes, respectively. Therefore, the OH P H 2 O 2 complex has a similar dissociation energy to the H 2 O P H 2 O and OH P H 2 O complexes. However, it is more tightly bound than the OH P H 2 S complex by 2.4 kcalrmol and less stable than the HO 2 P H 2 O complex by ; 2.8 kcalrmol. 3.3. Frequencies In Table 3, the harmonic vibrational frequencies and the corresponding infrared intensities for the OH P H 2 O 2 complex are shown. To compare with the vibrational frequencies of the isolated OH and H 2 O 2 , the frequency shifts of the complex relative to OH q H 2 O 2 are also listed. It is worth noting that although the UMP2 and UB3LYP methods yield similar geometries and energies for the OH P H 2 O 2 complex, they predict apparently different vibrational spectra. Inconsistent frequency shifts are also pre-
Table 2 Total energies Žin hartree. and binding energies Ž De and D 0 , in kcal moly1 . for the OH P H 2 O 2 complex Methods
OH P H 2 O 2
OH
H 2 O2
De
D0
MP2rrMP2 CCSDŽT.rrMP2 B3LYPrrB3LYP MP2rrB3LYP CCSDŽT.rrB3LYP
y226.88993 y226.93082 y227.37381 y226.88982 y226.93086
y75.59737 y75.61491 y75.76241 y75.59729 y75.61489
y151.28332 y151.30656 y151.60217 y151.28326 y151.30659
5.80 5.87 5.79 5.82 5.89
4.06 4.13 3.97 4.00 4.07
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
277
Table 3 Harmonic vibrational frequencies Žin cmy1 . and intensities Žin km moly1 . for the OH P H 2 O 2 complex at the UMP2Žfull.r6-311q q GŽd, p. and UB3LYPr6-311q q GŽd, p. levels No.
1 2 3 4 5 6 7 8 9 10 11 12 a b
Mode description
OH asym. stretching OH sym. stretching Y Y O H stretching OH sym. bending OH asym. bending X OO stretching intermolecular X X HOO H torsion intermolecular intermolecular intermolecular intermolecular
Frequenciesa
Intensitiesb
UMP2Žfull.
UB3LYP
UMP2Žfull.
UB3LYP
3 852.2 Žq0.1. 3 781.6 Žy69.5. 3 755.5 Žy87.2. 1 491.4 Žq32.6. 1 319.7 Žq17.9. 922.5 Žq0.1. 517.0 407.5 Žq12.7. 260.8 237.1 165.3 129.5
3 780.2 Žq3.2. 3 650.1 Žy125.9. 3 601.4 Žy105.5. 1 495.7 Žq42.0. 1 316.9 Žq20.4. 931.5 Žy3.1. 559.5 415.2 Žq48.3. 281.7 225.1 182.3 143.5
51.7 Ž3.2. 205.6 Ž2.9. 13.5 Ž0.7. 8.6 109.8 Ž1.1. 1.5 Ž1.4. 267.3 253.4 Ž1.1. 31.5 85.6 3.7 45.2
50.8 Ž3.8. 239.1 Ž3.5. 21.2 Ž1.6. 13.2 98.6 Ž1.1. 1.9 Ž1.7. 250.1 276.6 Ž1.2. 51.6 72.7 2.6 36.0
Numbers in parentheses are frequency shifts relative to the monomers. Numbers in parentheses are ratios of intensities for the complex relative to the monomers.
dicted. Because there are no experimental studies of the OH P H 2 O 2 complex, it is difficulty for us to determine which predictions should be more reliable. However, several important conclusions still can be drawn from the present high-level theoretical calculations. Both MP2 and B3LYP methods predict that there are four strong bands in the infrared spectrum of the OH P H 2 O 2 complex. They involve three intramolecular vibrational modes ŽHOOX HX torsion Ž n 8 ., OH symmetric stretching Ž n 2 ., and OH asymmetric bending Ž n 5 .. and one intermolecular vibrational mode Ž n 7 .. It is interesting to note that all three of these intramolecular modes belong to the H 2 O 2 moiety. To compare with the corresponding vibrational modes of the isolated H 2 O 2 molecule, the intensities of the n 8 and n 5 modes barely change while the intensity of n 2 mode increases by about three times. Both MP2 and B3LYP methods predict that the OY HY stretching mode Ž n 3 . in the OH moiety and the OH symmetric stretching mode Ž n 2 . in the H 2 O 2 moiety are significantly red-shifted. Another three bands in the H 2 O 2 moiety, OH asymmetric stretching Ž n 1 ., OH symmetric stretching Ž n4 ., and OH asymmetric bending Ž n 8 ., are predicted to be blueshifted. The largest blue-shift of 32.6–42.0 cmy1 results in the n4 mode. Moreover, this Raman active mode in the isolated H 2 O 2 molecule becomes weakly
infrared active in the complex with an intensity of 8.6–13.2 kmrmol. There are five intermolecular modes Ž n 7 , n 9 , n 10 , n 11 and n 12 .. For these new intermolecular modes the MP2 and B3LYP methods predict very similar vibrational fundamental frequencies and infrared intensities. Moreover, the n 7 mode is predicted to be one of the most intense bands in the infrared spectrum of the complex. The observation of these vibrational modes in the laboratory would support the existence of the OH P H 2 O 2 radical complex.
4. Conclusions The hydrogen bonding between the OH radical and the H 2 O 2 molecule has been studied using both UMP2Žfull. and UB3LYP methods with the 6-311 q q GŽd, p. basis set. The equilibrium structure of the OH P H 2 O complex is predicted to be a floppy five-membered ring involving two distorted O PPP H PPP O hydrogen bonds with O–O distances ˚ The binding energy D 0 of of ; 2.9 and ; 2.8 A. the OH P H 2 O 2 complex is calculated to be ; 4.1 kcalrmol at the CCSDŽT.r6-311 q q GŽ2d, 2p. level of theory. The infrared spectrum of the complex is also predicted.
B. Wang et al.r Chemical Physics Letters 309 (1999) 274–278
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x
A.C. Legon, D.J. Millen, Chem. Rev. 86 Ž1986. 635. A.C. Legon, D.J. Millen, Acc. Chem. Res. 20 Ž1987. 39. L.A. Curtiss, M. Blancker, Chem. Rev. 88 Ž1988. 963. P. Hobza, R. Zahradnik, Chem. Rev. 88 Ž1988. 871. D.A. Smith, Modeling the Hydrogen Bond, Am. Chem. Soc., Washington, DC, 1994. K.S. Kim, H.S. Kim, J.H. Jung, B.J. Mhin, Y. Xie, H.F. Schaefer, J. Chem. Phys. 94 Ž1991. 2057. Y. Xie, H.F. Schaefer, J. Chem. Phys. 98 Ž1993. 8829. B. Nelander, J. Phys. Chem. A 101 Ž1997. 9092. S. Aloisio, J.S. Francisco, J. Phys. Chem. A 102 Ž1998. 1899. B. Wang, H. Hou, Y. Gu, Chem. Phys. Lett. 303 Ž1999. 96. B. Wang, H. Hou, Y. Gu Žsubmitted.. H. Hippler, J. Troe, Chem. Phys. Lett. 192 Ž1992. 333. H. Hippler, H. Neunaber, J. Troe, J. Chem. Phys. 103 Ž1995. 3510.
w14x M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.W.M. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Allaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzales, J.A. Pople, Gaussian 94, Revision E.1, Gaussian, Inc., Pittsburgh, PA, 1995. w15x C. Møller, M.S. Plesset, Phys. Rev. 46 Ž1934. 618. w16x A.D. Becke, J. Chem. Phys. 98 Ž1993. 1372. w17x C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 41 Ž1988. 785. w18x J. Cizek, J. Chem. Phys. 45 Ž1966. 4256. w19x G.D. Purvis, R.J. Bartlett, J. Chem. Phys. 76 Ž1982. 1910. w20x J.A. Pople, M. Head-Gordon, K. Raghavachari, J. Chem. Phys. 87 Ž1987. 5968. w21x T.J. Lee, A.P. Rendell, J. Chem. Phys. 69 Ž1991. 399. w22x S.F. Boys, F. Bernardi, Mol. Phys. 19 Ž1970. 553.