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Electronic and molecular structure of the water dimer cation. A theoretical study
CilEhfICAL PHYSICS LETITEFG
Volume 95. number 6
ELECTRONIC
AND MOLECULAR
A THEORETICAL
STRUCTURE
18 hfarcll 1983
OF THE WATER DIMER CATION.
STUDY
Kenji SATO, Shinji TOMODA, Katsumi KIMURA Imritute for Molecdar
Science. Okazaki 444. Japan
and
Suehiro IWATA Department
of Chemismy, Keio L’niversity. Hiyoshi.
Yokoham
223. Japan
Received 22 December 1982
Ab initio configuration-interacrion calculations of the water dimer cation were performed with a basis set including pokuization functions. The ground and first excited states are energetically close to each other and have 1 *A” and 1 ‘A. symmetries_ with bent and linear H.-O-H equilibrium geometries, respectively_
1_ Introduction From recent He1 photoelectron spectroscopic work on the water dimer in a supersonic nozzle beam, Tomoda et al. [l] reached the following conclusions. (1) The first two photoelectron bands of the water dimer appear with maxima at 12.1 + 0.1 and 13.2 2 0.2 eV, corresponding to the first two vertical ionization energies_ (2) The first photoelectron band is very broad, showing a threshold at 11.1 eV which is regarded as the first adiabatic ionization energy. The threshold energy (11.1 eV) found from the photoelectron spectrum is consistent with that (11.21 eV) obtained by Ng et al_ [I] for the production of H,Of in their photoionization mass spectrometric study of the water dimer. The large energy difference between the adiabatic and the vertical ionization energy in the first photoelectron band of the water dimer is in remarkable contrast to the first, sharp photoelectron band of the water molecule in which the adiabatic and vertical ionization energies coincide [3] _Therefore, it has been suggested [l] that there should be a large change in equilibrium geometry between the neutral and the ionic state of the water dimer. According to electric-resonance spectroscopic work 0 009-2614/83/0000-0000/S
03.00
0 1983 North-Holland
by Dyke et al. [4,5], the water dimer in the ground state has a trans conformation with a nearly linear hydrogen bond. This trans linear structure of the water dimer is consistent with that predicted originally by Morokuma and Pedersen [6] from their ab initio calculations. For the water dimer cation, Lathan et al. [7] earlier indicated from their UHF MO calculations with the STO3G basis set that the proton is transferred to form a complex of the hydronium ion H30+ and the hydroxyl radical OH, assuming that the ground-state water dimer cation has 12A, symmetry (in C, point group)_ No experimental structure results have so far been published for the water dmier cation_ The H30+_._0H complex suggested by Lathan et al. [7] should have two low-lying states, 1 2A” and 1 ‘A’, which are energetically close to each other, since OH; is isoelectronic with NH3 and the ground state of OH is degenerate_ The 1 ‘A” and 1 2A’ states have an odd electron in the out-of-plane and in-plane orbitals, respectively- In the course of our ab initio calculations with the 6-31G basis set, we found that these two lowlying states are nearly degenerate at optimized geometries, but their total energies vary differently agpinst the angle LH__.O-H. Therefore, to clarify the situation, we considered it important to calculate the total ener579
~~>lUrn~
95, numbsr
6
18 Xlxrch 1953
CHEMICAL PHYSICS LETTERS
gies of tl~ese two states of the water dimer cation as a : tunction of the Lli...O-H angle, using a more sophislicated basis set including polarization functions. The purposes of the present work are (1) to make assigiments of tile experimental vertical ionization energies of rhe \vatcr dimer. (2 j to identify the low-lying rtdishatic ionic SI~WS of the water dimer. and (3) to study the ec!uilibriuin geometries of the first two ionic st3tes.
2.
~onlpl~tatiorta!
method
In Ihc prcscn~ work. ab initio contiguration-interxtion ((‘I ) calculations of the water dimer and its cation \verc‘ carried out at tile following three stages. Al! t!ie itm1pu13tions were performed at t!ie Cumputcr C‘rni~ crfthc Institute for hlolecular Science. ( I ) III order to evaluate vertical ionization energies (I,) for I!W water dimcr. xvc carried out SCF MO and (‘I ~al~ularions \vit!l rile 6-3IC basis set. using rhe svailable cspt‘rimcntsl geometry [5] _ The 1hlSl’iUi; progrsm ~2s used t;>r t!x SCF Xl0 calculations. A program profussed by I\~319 IS] was used for the CI calculations wllich are referred to liereafter as “Cl-l”. Full geometry aptimizations wit!1 tile 6-3 1G basis set were carried oul tiv tlw wafer dimrr and its cation. using tile SCF and 111I!- gradient methods. respectively. Similar optimiz:~ti(xls were also performed for 110. H20+. !130+ and 01 I. in order to evaluate the dissociation limit of the \v:itcr dimer carion. (2 1 Further Cl calculations were carried out for the \vater dimer cation on tile basis of the optimized geometries obt:!ined above (see fig. 1) using tile MID! 3* basis scI IQ] \vhis!i includes polarization functions. Valencei>.pc vai’3nl 510s us-cd here were determined from closed~1~11SC’!: cri!
Fig_ 1. Optimized _elromerries of tbc water dimcr cation in rhe (a) 1 ‘A” and (b) 1 ‘A* s!ales.
dissociation limit of the water dimer to Hz0 + HZ0 and those of the water dirner cation to HzO+ + Hz0 and H30+ + OH were evaluated from CI-2a and -7b calculations of HlO, HzO*. H30+ and OH. (3) The total energies of tile water dimer cation in the 1 ?A” and I ‘A’ states were also calculated as a function of the angle (0) between the hydrogen bond axis and tile OH radical. These CI calculations are re-
0.0
-
,
90
I20
150
180
8 IdqJ
1
210
240
270
Fig. 2. Calculated total energies of the water dimer cation in the 1 ‘A” and 1 ‘A’ srates. given as a function of the angle (0) between the hydrogen bond and the OH radical. The structural pxmmerers used are shown in the insert. The energy scale is given with respecr fo the minimum of the 1 2A” state (-15 1.7768 bartree).
Volunle 95, number 6
18 hlarch 1983
CHEMICAL PHYSICS LETTERS
.
ferred to hereafter as ‘W-3”, in which Dunning’s contracted GTFs with polarization functions [lo] were used as the basis set_ The MOs used here were obtained from closed-shell SCF MO calculations_ The electron configurations of single- and double-electron excitations with respect to the reference configurations were used, in which eleven electrons enter the two lowest vacant and the six highest occupied MOs. The geometrical parameters shown in fig. 2 were used, taken from experimentally available parametersof the hydronium ion H, O+ [ 111 and the hydroxyl radical OH [ 12]_ It was assumed that the hydrogen bond (0-H___O) is linear and the O_..O distance is 2.6 A.
3. Results and discussion Calculated I,, values of the water dimer obtained from the Cl-l calculations are summarized in table l_ It is seen from table 1 that the calculated 1, values for the first two vertical ionization energies are 0.8-l -0 eV lower than the experimental values, although the separarion (1.37 eV) between the first and the second calculated Iv values is in reasonable agreement with experiment (1 _l eV)_ The 1, values corrected for the difference between the experimental and the Cl value of HZ0 are also shown as CI-1* in table 1, in good agreement with experiment. Therefore, from the CI-1 cal-
culations it is safely concluded that the first and second photoelectron bands of the water dimer are attributed to the 1 2A” and 1 2A’ states of its cation_ Recently, Moncrieff et al. [ 131 reproduced the first and the second I,, of the water dimer within 0.4 eV from CI calculations including polarization functions_ The ma-m electron configurations of the ionic states obtained here from the CI-1 calculations are also summarized in table 1, indicating the following results. (I) The first vertical ionic state (1 ‘An) is largely attributed to the configuration of (Jlru)-’ in which $I0 is mainly due to the non-bonding orbital of the proton-donating HI0 component_ (2) In the second vertical ionic state (1 TA’), the main configuration is ($9)-l, but considerably mixed with (@8)-l, where the ninth and the eighth hlOs are mixtures of the in-plane non-bonding orbitals of the proton donor and acceptor with almost equal coefficients- (3) The third vertical ionic state is also a mixture of ($9)-l and (+8)-l *but has a different phase from the second vertical ionic state. The optimized geometries obtained here for the 1 ?A” and 1 ?A’ states are illustrated in fig_ 1. indicating that these ionic states are regarded.as the H,OH*_..OH complexes_ This is consistent with the earlier suggestion by Lathan et al_ [7] _The two structures in fig. 1 are similar to each other in the HZOH+ and OH components but considerably different in the angle D-I---O-H (145.6” in 1 ‘A”; 176.4” in 1 ?A’)_
Table 1 Ab initio rrsulrs on vertical ionization for the water dimer ________Y Ionic state
Experimental
Theoretical
I,(cW I, (ev) U-1 Iv (ev)
configuration
111
CI d)
this work (6-31G) CL1 * b)
liT c,
I, (eV)
Iv (eV)
1131
n)
l*A”
11.07
O-96(+,,)-’
1 l-99
13.73
11.7
111
1’A’
12.44
o-91($cJ)-’
- 0_31(+a)-t
13.36
14.08
13-l
13.2 i 0.2
+ O-1
Z2A’
13.24
0_91($r$’
+ 0.31(*o)-’
14.16
14.78
14.0
a) 9 to = 0_82o(2py)D, @o = 0_45o(2p~)~ + 0_43o(lps)D - 0_39@(2pz)D, 9s = 0_48@(3p~)~ - 0_4l@(?p~)~ - 0_34&!pz)DHere. the x axis is parallel to the O...O bond, the r axis is perpendicular to the Cs symmetry plane. and subscripts A and D indicate the proton acceptor and donor oxygen atoms, respectively- Atomic terms with coefficients larger than 0.20 are shown hereb)Iv(CI-I*) =1&I-1) + [ly(esp_) -I&I-l)] fiaO_ ‘) Koopmans- theorem_ d, Gaussian-type orbit& were used, including an osysen d polarization function and a hydrogen p function.
581
Volume 95. number 6
18 hfarch 1983
CHEWCAL PHYSICS LETTERS
Tablr I Results 01‘the Cl-2 calcutstions for the warer dimrr and its cation. compared with the experimental data. All values shown here are in rV __ .-._ _--I_ __--Theoretical
(1) 111s\\‘3tcr dimer in the ground state (1) dissociation limit to Hz0 + Hz0 t.3) cbc iirst ;Idiobafic ionic suite (4) the second adiabatic ionic stotc 1.5) dissociation limit to HsO+ + OH (6) dissociation limit to 1120+ + 1120 (7) dil’fcrcncc bct~~~n (6) rtrrd (5)
CI-Za
CI-Zb
0 0.16 9.84 10.00 10.98 11.00 1.02
0 0.37 9.71 9.83 11.08 11.96 0.88
:” l
component in each stale in fig. 1 shows 3 nearly planar structure. probably because of the neglect 01‘pulzrrizsrion functions in the present optimizations. Very recently. Curtiss [ I-11 1x1sperformed ab initio t:liF gccnllctry optimizations with the 6-3 lG* basis scr for tire \vatcr dhner cation. indicating that the first two adiabatic ionic states of the water dimer have the I I,01 I+ ...()I I structures with very nearly the same enrrgy. The uptirnized geomtries obtained by Curtiss 1I-l J 3re siniilar lo ours sb~~n in fig. 1_ ‘I‘l~c results ~~bt3incd from the Cl-2 calculations are suniniari~cd in table 2. including _ the dissociation limit and the adiabatic ionization cnrrgies of the water dimer aid the dissociation limits of the water dimer cation. It is seen from table 2 thaf a11these results are in reasonable ;ILJIL’~II~~I~I with the corresponding esperimental da13 as far as comparisons arc made. la ccntnection with the results in tables 1 and 2. tl~r follcnving points should be mentioned. (1) The t1i~~~1~tii31 value obtained from the present calculations for III~ difference between the first vertical and the first adiabatic ionization energy is about 1.2 eV. in good agreement \vitb rhe corresponding experimental flue. I .O e\’ ] I] _ (2) The theoretical value obtained from the Cl-?a (z111d Cl-2bj calculations for the differcncc bct\veen the dissociation limits to H,O+ + OH and I l,O+ + 1 I20 is 1.02 eV (0.8s eV). in good agreement with the corresponding experimental value. 1.1 1 c\’ (= 12.84 -. 1 1.73 eV: see table 2). The I I101 I+
Espcrimrntal
0 0.34 3) 11.1 b) 11.73 c) 12.84 d, 1.11
3) were used.
The 0 dependences of the total energies obtained from the Cl-3 calculations are illustrated in fig. 2. It is clearly seen from fig. 2 that the two curves behave quite differently_ The following conclusions have been deduced from fig. 2. (1) The 1 ‘A” state has double minima at =;120°
and 240°.
indicating
a bent
structure
at
equilibrium. (3) The 1 IA’ state has a broad minimum around 180”. indicating a linear structure at equilibrium_ (3) The ground state of the water dimer cation is assigned to 1 ?A”, 0.17 eV lower than the first excited state (1 ?A’). These are the most important conclusions obtained from the present work. It should be mentioned that the present assignment for the ground state of the water dimer cation differs from that reported earlier by Lathan et al. [7]. Finally. it should also be pointed out that the bent H...O-H conformation in the ground-state water dimer cation is consistent with the empirical rule that the proton attacks the proton accepting oxygen atom from the direction of either the sp’ or sp3 hybridized nonbonding orbital. On the other hand, it is interesting to note that the 1 “A’ state with one in-plane non-bonding electron has a linear H...O-H conformation.
Acknowledgement The authors would like to thank Dr. K. Kitaura.
S. Kato, rMr_U. Nagashima and Mr. M. Hanamura
Dr. for
Volume 95, number 6
CHEMICAL PHYSICS LLT’I-I-ERS
their helpful advice in performing the present ab initio caiculations. A part of this work has been carried out under the joint research program of IMS (SI).
References ( l] S. Tomoda, Y. Achiba and K. Kimura, Chem. Phys. Letters 87 (1982) 197. [2] C-Y. Ng. D-1. Trevor, P_\V_Tiedemann. ST. Ceyer, P-L_ Kronebusch. B.H. Mahan and Y-T_ Lee, J_ Chem. Phys. 67 (1977) 4235. [3] K. Kimura, S. Karsumata. Y. Achiba. T_ Yamazaki and S. Iwata, Handbook of He1 photoelectron spectra of fundamental organic molecules (Japan Scientific Societies Press/Halsted Press, Tokyo/New York, 1981) p_ 33. [4] T.R. Dyke, K.M. Mach and J-S. Muenter, 1. Chem. Phys. 66 (1977) 498.
18 March 1983
[S] J.A. Odutola and T-R. Dyke, J. Chem. Phys. 72 (1980) 5062. [6] K. hlorokumn and L. Pcdersen, J. Chem. Phys. 48 (1968) 3275. [7] W.A. Lathan. L-A. Curfiss, WJ. Hehre, J.B. Lisle and J-A. Pople, Progr. Phys. Org. Chem. 11 (1974) 175. IS] S. Iwata, Chem. Phys. Letters 83 (1981) 134. [9] H. Tatewaki and S. Huzinaga, J. Comput. Chem. 1 (1980) 205. [lo] T-H. Dunning Jr. and PJ. Hay. in: hlodem theoretical chemistry, Vol. 3, ed. by H-F. Schaefer III (Plenum Press. New York, 1976) p_ l_ [ 1l] R. Savoie and P-A. Giguire. J. Chem. Phys. 41 (1964) 2698. [ 121 %f. hfizushima,Phys. Rev. A5 (1972) 143. [ 131 D. Moncrieff. 1-H. Hillier and V-R. Saunders, Chcm. Phys. Letters 89 (1981) 4.47. ] 14 ] L.A. Curtiss, to be published. [ 151 L-A. Curtiss, DJ_ Frurip and hl. Blander. J. Chem. Ph>-s. 71 (1979) 2703.
583
Volume 95. number 6
ELECTRONIC
AND MOLECULAR
A THEORETICAL
STRUCTURE
18 hfarcll 1983
OF THE WATER DIMER CATION.
STUDY
Kenji SATO, Shinji TOMODA, Katsumi KIMURA Imritute for Molecdar
Science. Okazaki 444. Japan
and
Suehiro IWATA Department
of Chemismy, Keio L’niversity. Hiyoshi.
Yokoham
223. Japan
Received 22 December 1982
Ab initio configuration-interacrion calculations of the water dimer cation were performed with a basis set including pokuization functions. The ground and first excited states are energetically close to each other and have 1 *A” and 1 ‘A. symmetries_ with bent and linear H.-O-H equilibrium geometries, respectively_
1_ Introduction From recent He1 photoelectron spectroscopic work on the water dimer in a supersonic nozzle beam, Tomoda et al. [l] reached the following conclusions. (1) The first two photoelectron bands of the water dimer appear with maxima at 12.1 + 0.1 and 13.2 2 0.2 eV, corresponding to the first two vertical ionization energies_ (2) The first photoelectron band is very broad, showing a threshold at 11.1 eV which is regarded as the first adiabatic ionization energy. The threshold energy (11.1 eV) found from the photoelectron spectrum is consistent with that (11.21 eV) obtained by Ng et al_ [I] for the production of H,Of in their photoionization mass spectrometric study of the water dimer. The large energy difference between the adiabatic and the vertical ionization energy in the first photoelectron band of the water dimer is in remarkable contrast to the first, sharp photoelectron band of the water molecule in which the adiabatic and vertical ionization energies coincide [3] _Therefore, it has been suggested [l] that there should be a large change in equilibrium geometry between the neutral and the ionic state of the water dimer. According to electric-resonance spectroscopic work 0 009-2614/83/0000-0000/S
03.00
0 1983 North-Holland
by Dyke et al. [4,5], the water dimer in the ground state has a trans conformation with a nearly linear hydrogen bond. This trans linear structure of the water dimer is consistent with that predicted originally by Morokuma and Pedersen [6] from their ab initio calculations. For the water dimer cation, Lathan et al. [7] earlier indicated from their UHF MO calculations with the STO3G basis set that the proton is transferred to form a complex of the hydronium ion H30+ and the hydroxyl radical OH, assuming that the ground-state water dimer cation has 12A, symmetry (in C, point group)_ No experimental structure results have so far been published for the water dmier cation_ The H30+_._0H complex suggested by Lathan et al. [7] should have two low-lying states, 1 2A” and 1 ‘A’, which are energetically close to each other, since OH; is isoelectronic with NH3 and the ground state of OH is degenerate_ The 1 ‘A” and 1 2A’ states have an odd electron in the out-of-plane and in-plane orbitals, respectively- In the course of our ab initio calculations with the 6-31G basis set, we found that these two lowlying states are nearly degenerate at optimized geometries, but their total energies vary differently agpinst the angle LH__.O-H. Therefore, to clarify the situation, we considered it important to calculate the total ener579
~~>lUrn~
95, numbsr
6
18 Xlxrch 1953
CHEMICAL PHYSICS LETTERS
gies of tl~ese two states of the water dimer cation as a : tunction of the Lli...O-H angle, using a more sophislicated basis set including polarization functions. The purposes of the present work are (1) to make assigiments of tile experimental vertical ionization energies of rhe \vatcr dimer. (2 j to identify the low-lying rtdishatic ionic SI~WS of the water dimer. and (3) to study the ec!uilibriuin geometries of the first two ionic st3tes.
2.
~onlpl~tatiorta!
method
In Ihc prcscn~ work. ab initio contiguration-interxtion ((‘I ) calculations of the water dimer and its cation \verc‘ carried out at tile following three stages. Al! t!ie itm1pu13tions were performed at t!ie Cumputcr C‘rni~ crfthc Institute for hlolecular Science. ( I ) III order to evaluate vertical ionization energies (I,) for I!W water dimcr. xvc carried out SCF MO and (‘I ~al~ularions \vit!l rile 6-3IC basis set. using rhe svailable cspt‘rimcntsl geometry [5] _ The 1hlSl’iUi; progrsm ~2s used t;>r t!x SCF Xl0 calculations. A program profussed by I\~319 IS] was used for the CI calculations wllich are referred to liereafter as “Cl-l”. Full geometry aptimizations wit!1 tile 6-3 1G basis set were carried oul tiv tlw wafer dimrr and its cation. using tile SCF and 111I!- gradient methods. respectively. Similar optimiz:~ti(xls were also performed for 110. H20+. !130+ and 01 I. in order to evaluate the dissociation limit of the \v:itcr dimer carion. (2 1 Further Cl calculations were carried out for the \vater dimer cation on tile basis of the optimized geometries obt:!ined above (see fig. 1) using tile MID! 3* basis scI IQ] \vhis!i includes polarization functions. Valencei>.pc vai’3nl 510s us-cd here were determined from closed~1~11SC’!: cri!
Fig_ 1. Optimized _elromerries of tbc water dimcr cation in rhe (a) 1 ‘A” and (b) 1 ‘A* s!ales.
dissociation limit of the water dimer to Hz0 + HZ0 and those of the water dirner cation to HzO+ + Hz0 and H30+ + OH were evaluated from CI-2a and -7b calculations of HlO, HzO*. H30+ and OH. (3) The total energies of tile water dimer cation in the 1 ?A” and I ‘A’ states were also calculated as a function of the angle (0) between the hydrogen bond axis and tile OH radical. These CI calculations are re-
0.0
-
,
90
I20
150
180
8 IdqJ
1
210
240
270
Fig. 2. Calculated total energies of the water dimer cation in the 1 ‘A” and 1 ‘A’ srates. given as a function of the angle (0) between the hydrogen bond and the OH radical. The structural pxmmerers used are shown in the insert. The energy scale is given with respecr fo the minimum of the 1 2A” state (-15 1.7768 bartree).
Volunle 95, number 6
18 hlarch 1983
CHEMICAL PHYSICS LETTERS
.
ferred to hereafter as ‘W-3”, in which Dunning’s contracted GTFs with polarization functions [lo] were used as the basis set_ The MOs used here were obtained from closed-shell SCF MO calculations_ The electron configurations of single- and double-electron excitations with respect to the reference configurations were used, in which eleven electrons enter the two lowest vacant and the six highest occupied MOs. The geometrical parameters shown in fig. 2 were used, taken from experimentally available parametersof the hydronium ion H, O+ [ 111 and the hydroxyl radical OH [ 12]_ It was assumed that the hydrogen bond (0-H___O) is linear and the O_..O distance is 2.6 A.
3. Results and discussion Calculated I,, values of the water dimer obtained from the Cl-l calculations are summarized in table l_ It is seen from table 1 that the calculated 1, values for the first two vertical ionization energies are 0.8-l -0 eV lower than the experimental values, although the separarion (1.37 eV) between the first and the second calculated Iv values is in reasonable agreement with experiment (1 _l eV)_ The 1, values corrected for the difference between the experimental and the Cl value of HZ0 are also shown as CI-1* in table 1, in good agreement with experiment. Therefore, from the CI-1 cal-
culations it is safely concluded that the first and second photoelectron bands of the water dimer are attributed to the 1 2A” and 1 2A’ states of its cation_ Recently, Moncrieff et al. [ 131 reproduced the first and the second I,, of the water dimer within 0.4 eV from CI calculations including polarization functions_ The ma-m electron configurations of the ionic states obtained here from the CI-1 calculations are also summarized in table 1, indicating the following results. (I) The first vertical ionic state (1 ‘An) is largely attributed to the configuration of (Jlru)-’ in which $I0 is mainly due to the non-bonding orbital of the proton-donating HI0 component_ (2) In the second vertical ionic state (1 TA’), the main configuration is ($9)-l, but considerably mixed with (@8)-l, where the ninth and the eighth hlOs are mixtures of the in-plane non-bonding orbitals of the proton donor and acceptor with almost equal coefficients- (3) The third vertical ionic state is also a mixture of ($9)-l and (+8)-l *but has a different phase from the second vertical ionic state. The optimized geometries obtained here for the 1 ?A” and 1 ?A’ states are illustrated in fig_ 1. indicating that these ionic states are regarded.as the H,OH*_..OH complexes_ This is consistent with the earlier suggestion by Lathan et al_ [7] _The two structures in fig. 1 are similar to each other in the HZOH+ and OH components but considerably different in the angle D-I---O-H (145.6” in 1 ‘A”; 176.4” in 1 ?A’)_
Table 1 Ab initio rrsulrs on vertical ionization for the water dimer ________Y Ionic state
Experimental
Theoretical
I,(cW I, (ev) U-1 Iv (ev)
configuration
111
CI d)
this work (6-31G) CL1 * b)
liT c,
I, (eV)
Iv (eV)
1131
n)
l*A”
11.07
O-96(+,,)-’
1 l-99
13.73
11.7
111
1’A’
12.44
o-91($cJ)-’
- 0_31(+a)-t
13.36
14.08
13-l
13.2 i 0.2
+ O-1
Z2A’
13.24
0_91($r$’
+ 0.31(*o)-’
14.16
14.78
14.0
a) 9 to = 0_82o(2py)D, @o = 0_45o(2p~)~ + 0_43o(lps)D - 0_39@(2pz)D, 9s = 0_48@(3p~)~ - 0_4l@(?p~)~ - 0_34&!pz)DHere. the x axis is parallel to the O...O bond, the r axis is perpendicular to the Cs symmetry plane. and subscripts A and D indicate the proton acceptor and donor oxygen atoms, respectively- Atomic terms with coefficients larger than 0.20 are shown hereb)Iv(CI-I*) =1&I-1) + [ly(esp_) -I&I-l)] fiaO_ ‘) Koopmans- theorem_ d, Gaussian-type orbit& were used, including an osysen d polarization function and a hydrogen p function.
581
Volume 95. number 6
18 hfarch 1983
CHEWCAL PHYSICS LETTERS
Tablr I Results 01‘the Cl-2 calcutstions for the warer dimrr and its cation. compared with the experimental data. All values shown here are in rV __ .-._ _--I_ __--Theoretical
(1) 111s\\‘3tcr dimer in the ground state (1) dissociation limit to Hz0 + Hz0 t.3) cbc iirst ;Idiobafic ionic suite (4) the second adiabatic ionic stotc 1.5) dissociation limit to HsO+ + OH (6) dissociation limit to 1120+ + 1120 (7) dil’fcrcncc bct~~~n (6) rtrrd (5)
CI-Za
CI-Zb
0 0.16 9.84 10.00 10.98 11.00 1.02
0 0.37 9.71 9.83 11.08 11.96 0.88
:” l
component in each stale in fig. 1 shows 3 nearly planar structure. probably because of the neglect 01‘pulzrrizsrion functions in the present optimizations. Very recently. Curtiss [ I-11 1x1sperformed ab initio t:liF gccnllctry optimizations with the 6-3 lG* basis scr for tire \vatcr dhner cation. indicating that the first two adiabatic ionic states of the water dimer have the I I,01 I+ ...()I I structures with very nearly the same enrrgy. The uptirnized geomtries obtained by Curtiss 1I-l J 3re siniilar lo ours sb~~n in fig. 1_ ‘I‘l~c results ~~bt3incd from the Cl-2 calculations are suniniari~cd in table 2. including _ the dissociation limit and the adiabatic ionization cnrrgies of the water dimer aid the dissociation limits of the water dimer cation. It is seen from table 2 thaf a11these results are in reasonable ;ILJIL’~II~~I~I with the corresponding esperimental da13 as far as comparisons arc made. la ccntnection with the results in tables 1 and 2. tl~r follcnving points should be mentioned. (1) The t1i~~~1~tii31 value obtained from the present calculations for III~ difference between the first vertical and the first adiabatic ionization energy is about 1.2 eV. in good agreement \vitb rhe corresponding experimental flue. I .O e\’ ] I] _ (2) The theoretical value obtained from the Cl-?a (z111d Cl-2bj calculations for the differcncc bct\veen the dissociation limits to H,O+ + OH and I l,O+ + 1 I20 is 1.02 eV (0.8s eV). in good agreement with the corresponding experimental value. 1.1 1 c\’ (= 12.84 -. 1 1.73 eV: see table 2). The I I101 I+
Espcrimrntal
0 0.34 3) 11.1 b) 11.73 c) 12.84 d, 1.11
3) were used.
The 0 dependences of the total energies obtained from the Cl-3 calculations are illustrated in fig. 2. It is clearly seen from fig. 2 that the two curves behave quite differently_ The following conclusions have been deduced from fig. 2. (1) The 1 ‘A” state has double minima at =;120°
and 240°.
indicating
a bent
structure
at
equilibrium. (3) The 1 IA’ state has a broad minimum around 180”. indicating a linear structure at equilibrium_ (3) The ground state of the water dimer cation is assigned to 1 ?A”, 0.17 eV lower than the first excited state (1 ?A’). These are the most important conclusions obtained from the present work. It should be mentioned that the present assignment for the ground state of the water dimer cation differs from that reported earlier by Lathan et al. [7]. Finally. it should also be pointed out that the bent H...O-H conformation in the ground-state water dimer cation is consistent with the empirical rule that the proton attacks the proton accepting oxygen atom from the direction of either the sp’ or sp3 hybridized nonbonding orbital. On the other hand, it is interesting to note that the 1 “A’ state with one in-plane non-bonding electron has a linear H...O-H conformation.
Acknowledgement The authors would like to thank Dr. K. Kitaura.
S. Kato, rMr_U. Nagashima and Mr. M. Hanamura
Dr. for
Volume 95, number 6
CHEMICAL PHYSICS LLT’I-I-ERS
their helpful advice in performing the present ab initio caiculations. A part of this work has been carried out under the joint research program of IMS (SI).
References ( l] S. Tomoda, Y. Achiba and K. Kimura, Chem. Phys. Letters 87 (1982) 197. [2] C-Y. Ng. D-1. Trevor, P_\V_Tiedemann. ST. Ceyer, P-L_ Kronebusch. B.H. Mahan and Y-T_ Lee, J_ Chem. Phys. 67 (1977) 4235. [3] K. Kimura, S. Karsumata. Y. Achiba. T_ Yamazaki and S. Iwata, Handbook of He1 photoelectron spectra of fundamental organic molecules (Japan Scientific Societies Press/Halsted Press, Tokyo/New York, 1981) p_ 33. [4] T.R. Dyke, K.M. Mach and J-S. Muenter, 1. Chem. Phys. 66 (1977) 498.
18 March 1983
[S] J.A. Odutola and T-R. Dyke, J. Chem. Phys. 72 (1980) 5062. [6] K. hlorokumn and L. Pcdersen, J. Chem. Phys. 48 (1968) 3275. [7] W.A. Lathan. L-A. Curfiss, WJ. Hehre, J.B. Lisle and J-A. Pople, Progr. Phys. Org. Chem. 11 (1974) 175. IS] S. Iwata, Chem. Phys. Letters 83 (1981) 134. [9] H. Tatewaki and S. Huzinaga, J. Comput. Chem. 1 (1980) 205. [lo] T-H. Dunning Jr. and PJ. Hay. in: hlodem theoretical chemistry, Vol. 3, ed. by H-F. Schaefer III (Plenum Press. New York, 1976) p_ l_ [ 1l] R. Savoie and P-A. Giguire. J. Chem. Phys. 41 (1964) 2698. [ 121 %f. hfizushima,Phys. Rev. A5 (1972) 143. [ 131 D. Moncrieff. 1-H. Hillier and V-R. Saunders, Chcm. Phys. Letters 89 (1981) 4.47. ] 14 ] L.A. Curtiss, to be published. [ 151 L-A. Curtiss, DJ_ Frurip and hl. Blander. J. Chem. Ph>-s. 71 (1979) 2703.
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