Conformers and IntramolecularHydrogen Bonding of the Oxalic AcidMonomer and Its Anions

CHENG CHEN, SHUANG-FUH SHYUDepartment of Applied Chemistry, Chung-Cheng Institute of Technology, Ta-Hsi, Taoyuan 33509,Taiwan, Republic of China

Received 10 June 1997; revised 21 June 1999; accepted 30 June 1999

Ž .ABSTRACT: Density functional theory, B3LYPr6-31G** and B3LYPr6-311 q G 2d,p ,and ab initio MP2r6-31G** calculations have been carried out to investigate theconformers, transition states, and energy barriers of the conformational processes ofoxalic acid and its anions. QCISDr6-31G** geometrical optimization is also performed inthe stable forms. Its calculated energy differences between the two most stable conformersare very near to the related observed value at 7.0 kJrmol. It is found that the structuresand relative energies of oxalic acid conformers predicted by these methods show similar

Ž .results, and that the conformer L1 C with the double-interfunctional-groups hydrogen2hbonds is the most stable conformer. The magnitude of hydrogen bond energies dependson the energy differences of various optimized structures. The hydrogen bond energieswill be about 32 kJrmol for interfunctional groups, 17 kJrmol for weak interfunctional

Ž .groups, 24 kJrmol for intra-COOH in COOH , and 60 kJrmol for interfunctional2Ž . y1 Ž .groups in COOH COO ion if calculated using the B3LYPr6-311 q G 2d,p method.

Q 2000 John Wiley & Sons, Inc. Int J Quant Chem 76: 541]551, 2000

Key words: oxalic acid; interfunctional hydrogen bond; intrafunctional hydrogen bond

Introduction

he small molecule of oxalic acid is composedT of two —COOH groups. But, the, varioustypes of orientations between different functional

Correspondence to: C. Chen.Contract grant sponsor: National Science Council of the

Republic of China.Contract grant number: Nsc 88-2113-m-014-002.

groups may have the conformers formed in differ-ent types of bonding effect. The most importantbonding in the oxalic acid system is the interfunc-tional hydrogen bond, which has been reported in

w xseveral experimental 1]7 and theoretical studiesw x7]13 . From our recent theoretical study of the

Žsimple carboxylic acid RCOOH R s H, CH ,3.C H , C H , etc. , by using MP2r6-31G** or2 5 6 5

w xB3LYPr6-31G** calculations for various R 14, 15 ,it was found that the energy differences between

( )International Journal of Quantum Chemistry, Vol. 76, 541]551 2000Q 2000 John Wiley & Sons, Inc. CCC 0020-7608 / 00 / 040541-11

CHEN AND SHYU

the optimized structures

O O

R C H and R C OO H

are more than 22 kJrmol. Though the interfunc-tional groups with a five-membered ring hydrogenbond in oxalic acid is considered as importantin intramolecular hydrogen bonding, we strongly

Ž .recommend here the intra- COOH group four-membered ring-type hydrogen bond, which playsan important role also. In order to interpret thesetwo kinds of hydrogen bonds for various oxalicsystems, we selected various conformers and theirrelated transition states to achieve a theoreticalstudy.

Calculation Methods

The quantum chemical calculations performedin this work have been done with the Gaussian 94

w x w xprogram 16 . MP2r6-31G** 17 included correla-tion energy via second-order Moller—Plesset per-

w xturbation theory. B3LYPr6-31G** 18]19 andŽ . ŽB3LYPr6-311 q G 2d,p Beckes three-parameter

.hybrid method used the density functional the-ory. Those methods were applied in this study toall the energy, geometry, and frequency optimiza-

Ž .tion type of calculations of the COOH ,2Ž . y1 Ž .y2COOH COO , and COO systems. In order to2determine the internal rotational transition states

Ž .of COOH , we applied either the QST3-type op-2timization procedure from the Gaussian 94 pro-gram or the symmetry-regulated-type optimi-

Ž .zation method. QCISDr6-31G** Quadratic CIw xcalculation 20 , including single- and double-

substitution-type geometrical optimization, wasalso preformed.

Also, in order to understand which type ofintramolecular hydrogen bonding was more im-portant in the molecular systems and ignoring theother type effects in geometry, we calculated theenergy differences of various structures, hoping toestimate the related hydrogen bond energies.

Calculation and Result

LOCAL MINIMA AND RELATIVE ENERGIESOF CONFORMERS

Ž .Six local minima existing in COOH were2identified through MP2r6-31G** and B3LYPr6-

31G** methods, as shown in Figure 1. We definedthem as L1, L2, L3, . . . etc., according to theirmolecular stability. The relative energies are listed

Ž .in Table I for comparison. For the COOH sys-2tem, L1, the most stable conformer, is formed with

Ž .Ž .two inter- COOH COOH group hydrogen bondswith C structure. Form L2 comprises one inter-2 hŽ . Ž . Ž .COOH COOH and one intra- COOH hydrogen

Ž .bond in the C planar structure. Forms L3 Cs 2 hŽ .and L4 C are both formed with two intra-2 v

Ž . Ž .COOH hydrogen bonds. Form L5 C is the lastsŽ .planar structure of COOH . Similar to L2, L5 is2

Ž .also formed by one intra- COOH and one inter-Ž .Ž .COOH COOH hydrogen bond. However, since

Ž .Ž .the inter- COOH COOH hydrogen bond is con-Ž .structed by two— OH groups, rather than

one—OH and one—C5O in the case of L2, themolecular energy of L5 is higher than that of L2.Because there is no intramolecular hydrogen bond-ing and because of the C5O, C5O and OH, OHrepulsion, L6, the only nonplanar structure, be-comes the most unstable conformer with C sym-2metry, which results its energy being significantlyhigher than those of other conformers. In order toget more information about the correlation energythan the above-mentioned results, we selected theQCISDr6-31G** geometrical optimization methodto calculate the energies of the conformers. Unfor-tunately, the memory space of G94 limits the cal-culation of the frequencies and zero-point energyby this software. However, since optimized ge-ometries of QCISDr6-31G** and MP2r6-31G**methods are very close, we assume that the zero-point energy corrections of these two methods are

Ž .very close also. Thus, we predict the E 0 K ofQCISDr6-31G** according to the zero-point en-ergy of MP2r6-31G** and make corresponding

Ž .correction. From this result of E 0 K energy differ-ences of L1 and L2, seen in the Table I, it is easilyfound that the calculated energy differences ofthese energies are very close to the experimentallyobserved value of 7.0 kJrmol, as reported in Ref. 6.

Ž .The B3LYPr6-311 q G 2d,p calculation only iden-tifies five local minima. Minima L1, L2, L3, and L5are planar structures and L4q with nonplanar C2symmetry.

Interestingly, results of these four methods areŽvery similar. Two local minima both in the Cs

. Ž . y1group have been found for the COOH COOion. That is, L1y1 is a planar structure with inter-functional group hydrogen bonded, and L2y1 is of

Ž .the nonplanar intra- COOH hydrogen bondedŽ .y2type. Besides, the COO dianion structure with-2

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( ) ( ) ( ) ( )FIGURE 1. Local minima of COOH and its anion structures: a MP2 / 6-31G** data; b B3LYP / 6-31G** data; c2( ) ( )B3LYP / 6-311 + G 2d,p data; d QCISD / 6-31G** data.

out hydrogen bonding is the only stable conformerwith nonplanar D -type staggered form.2 d

TRANSITION STATE STRUCTURES ANDBARRIERS OF INTERNAL ROTATION

Ž .In the COOH system, except for the higher2Ž . Ž .energy planar structures, T66 C , T44 C , and2 v 2 v

qŽ .T34 C , which were calculated by symmetry-2fixed-type optimization of the B3LYPr6-311 qŽ .G 2d,p method, other transition states were calcu-

lated using the QST3-type optimization method.Here, Tmn was defined as the transition statebetween Lm and Ln. Following execution of thevarious optimization procedures, the frequency ofeach conformer was calculated. In case of local

minima, all the frequencies would be real andpositive, and in the case of transition states, oneimaginary frequency would have a saddle point.

Ž .All the calculated E and the energies E 0 KSCFof the corrected zero-point energy are tabulated inTable I. Transition states of planar type, T44 andT66, were searched via internal rotation of L4 andL6, respectively. All the optimized geometrical dataof transition states are plotted in Figure 2.

Ž .In Table I and Figure 2, the energies E 0 Kcorresponding to the local minima and transitionstates were used to determine the potential barri-ers, as shown in Table II and Scheme 1. In the

Ž .B3LYPr6-311 q G 2d,p method, L1 can rotate toL2 through the T12 transition state or to the T66transition state. By means of the C—C bond rota-

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 543

CHEN AND SHYU

TABLE Ia ( ) ( )Energies of equilibrium and transition structures of COOH and its anions in kJ ///// mol .2

( )MP2 / 6-31G** B3LYP / 6-31G** B3LYP / 6-311 + G 2d,p QCISD / 6-31G**b( ) ( ) ( ) ( )Form E E 0 K E E 0 K E E 0 K E E 0 KSCF SCF SCF SCF

L1 y377.348296 y377.298181 y378.330681 y378.280928 y378.464892 y378.415402 y377.362836L2 8.3 7.7 10.9 10.1 11.2 10.3 7.6 7.0L3 10.3 9.0 16.6 15.2 17.90 16.0 9.6 8.4

cL4 12.1 10.6 18.4 16.7 17.91 16.0 11.5 9.9L5 20.7 18.7 25.2 22.9 25.4 22.8 20.0 17.9L6 65.0 61.4 67.3 63.5 62.9 59.3T12 62.7 56.8 63.5 57.6 60.4 54.5T23 62.5 56.2 67.1 60.5 63.1 56.3

dT34 17.0 14.9 22.7 20.1 18.1 16.1cT44 19.9 17.7

T45 65.0 58.3 70.5 63.6T56 76.6 70.0 79.4 72.7T16 67.9 63.3 68.9 64.3T25 41.9 38.5 44.8 41.2 38.0 34.4

eT66 91.1 83.4 90.2 82.5 84.2 76.9y1L1 y376.805318 y376.768788 y377.786825 y377.750703 y377.941735 y377.905740 y376.814659y1L2 52.5 51.3 51.8 50.3 37.6 35.9 45.8 44.6

T1y1 54.6 52.9 54.7 52.9 36.0 34.2 48.8 47.1y1T2 56.5 55.5 58.9 57.7 51.4 49.6 52.9 51.9

y2L y376.035203 y376.011304 y377.014659 y376.991141 y377.221199 y377.198190 y376.044305y2T 11.8 11.3 11.7 11.4 21.9 23.3 16.0 15.5

aRelative energies with respect to L1, L1y 1, and Ly 2 in kJ / mol. For L1, L1y 1 the energies are given in a.u.b Zero-point energy corrections are chosen from the calculated results of MP2 / 6-31G** method.cmeans L4+ form.dMeans T34+ transition structure.ePlanar transition structure.

tion, L2 can rotate to L5 through T25, easier thanto L3 through T23 by means of the C—O bondrotation. The former value is 24.1 kJrmol and the

Ž . Ž .latter is 46.0 kJrmol Scheme 1 . When L3 C2 hq Ž . qrotates to L4 C and L4 rotates to T44 through2

the C—C bond rotation, their rotational barriersŽ .are very small 0.1 and 1.7 kJrmol . But, when L1Žrotates to T66 through breaking two interfunc-

.tional group hydrogen bonds and L2 rotates to L3Žthrough breaking one interfunctional group hy-

.drogen bond , their rotational barriers are 76.9 and46.0 kJrmol. It is obviously proved that the inter-functional group hydrogen bonding affects the sta-bility of the conformer.

Two internal rotational transition states, T1y1

y1 Ž . y1and T2 , are found in the COOH COO ion,both of which have C -type structures. The struc-sture T1y1 is nonplanar with the —COOH planeperpendicular to the —CO plane, but T2y1 is a2

Ž .y2planar structure. In the dianion COO struc-2ture, only one internal rotation transition state

Ž y2 .T with D planar structure has been found.2 hy1 ] 1 Ž . y1 y2Both T1 and T2 of COOH COO and T of

Ž .y2COO can be easily obtained during the sym-2metry-regulated-type optimization procedure.

COMPARISON OF THECONFORMATIONAL STRUCTURES

Ž .If the optimized local minima of COOH and2Ž . y1COOH COO ion are considered, it is obviousthat the comparatively stable structures are mostlycomprised of planar structure and intramolecular

Ž .y2hydrogen bonding. But, in the COO ion, no2hydrogen atom is involved in the molecular sys-tem, and hence no hydrogen bonding will occur. Ithas only one stable local minimum, that is, D -2 dtype nonplanar structure. Based on the said geo-metrical results, it is not difficult to realize that theformation of planar structures is mainly caused by

Ž .hydrogen bonding. In the COOH molecule, five2Ž .stable conformers L1]L5 with planar structures

VOL. 76, NO. 4544

CONFORMER AND INTRAMOLECULAR HYDROGEN BONDING

( ) ( y10 )FIGURE 2. Transition states of COOH and its anion structural parameter bond distances in 10 m and angles2( ) ( )in degrees calculated by B3LYP / 6-311 + G 2d,p method.

and the only nonplanar highest energy conformer,L6, without hydrogen bonding, have been foundwith the B3LYPr6-31G**, MP2r6-31G**, andQCISDr6-31G** methods. Among these five pla-nar conformers, L1, with two interfunctional grouphydrogen bonds, is the most stable. Form L2 is thesecond because it has one interfunctional grouphydrogen bond and one intra-COOH hydrogenbond. Forms L3 and L4, both of which consist of

Ž .two intra-COOH four-membered rings type hy-drogen bonds, are very close in molecular energy.Since the internal rotation barriers of the O—C—C

—O dihedral angle variation are very low, theymay not be easily observed experimentally. Though

Ž .one inter- OH -type hydrogen bond and one in-2Ž .tra- COOH -type hydrogen bond have L5 formed

Ž .a planar structure, the inter- OH -type hydrogen2Ž .bond is not as stable as the inter- OH and

Ž .—C5O -type hydrogen bond. In the viewpoint ofmolecular energy, L5 is even less stable than the

Ž .pure intra- COOH hydrogen-bonded L3 and L4.The interfunctional-group-type planar con-

Ž . y1 y1former in the COOH COO ion, L1 , is themost stable structure. Corresponding to T1y1 and

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 545

CHEN AND SHYUTA

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T2y1, its internal rotation barriers are about 34]53and 50]58 kJrmol. The high-energy conformer,

y1 Ž .L2 , possesses a nonplanar intra- COOH hydro-gen-bonded-type structure, higher than L1y1 atabout 36]51 kJrmol in energy. The potential en-ergy barriers between L2y1 and T1y1 and betweenL2y1 and T2y1 are very small, which are y1.7]2.6

Žand 4]14 kJrmol, respectively see Table II and.Scheme 1 . The negative energy gap, which oc-

curred due to the differences of zero-point energycorrection between 3n y 6 n of L2y and 3n y 7ireal n of T1y1, is not reliable. Therefore, withi

Ž .Ž y. y1inter- COOH COO hydrogen bonding in L1 ,Ž . y1it is the only stable conformer in COOH COO ,

Ž .while the intra- COOH hydrogen bond nonplanartype is significantly less stable than L1y1, fromboth molecular energy and potential barrier pointsof view.

Ž .y2As for the non-hydrogen-bonded COO ion,2both the geometrical structure and potential bar-

Ž .rier are quite different from those of COOH and2Ž . y1 y2 Ž .y2COOH COO . Form L of COO structure,2with nonplanar D , is the only stable conformer,2 dand D -type planar Ty2 is the only internal rota-2 htion transition state with one imaginary frequencyfor this dianion. The potential barrier between Ly2

and T 2, as shown in Table II, is only 11]23 kJrmol,which is much less than the hydrogen-bonded-type

Ž . Ž . y1energy barriers of COOH and COOH COO .2

BOND DISTANCES AND ANGLES IN THEVARIOUS CONFORMERS ABOUTHYDROGEN BONDS

In order to compare the geometrical effects ofvarious types of hydrogen bonding, the distancesand angles of the hydrogen bonding are summa-rized in Table III. It can be clearly seen that both

Ž .Ž .d and d in the inter- COOH COOH hy-C 5 O O — Hdrogen bonds are longer than the corresponding

Ž .distances of the intra- COOH hydrogen bonds,and the shortest d and d belong to the L6C 5 O O — Hwithout hydrogen bonding. The C5O distance,wherein O connects with inter- and intra-hydrogen

˚Ž .bonds in L2, is the longest 1.208]1.224 A in theŽ .COOH system. Due to the compensational ef-2fect, d in C—OH will be longer than usualC — Owhen d is shorter. Among all the d ofO — H C — O

Ž .various COOH conformers, L6 has the longest2˚Ž . Žd 1.362]1.366 A and the shortest d 0.966C — O O — H

˚ y1. Ž .]0.970 A . In the COOH COO ion, because the˚Ž .optimized d 1.278]1.284 A and dC 5 O O — H

˚ y1Ž .0.995]1.018 A of L1 are very specially elon-

VOL. 76, NO. 4546

CONFORMER AND INTRAMOLECULAR HYDROGEN BONDING

( )SCHEME 1. Conformational mechanism of oxalic acid and its anions, barriers in kJ / mol calculated by B3LYP /( )6-311 + G 2d,p plus zero-point vibrational energy.

gated to about 0.03 and 0.01, respectively, it ispredicted that L1y1 has the strong inter-Ž .Ž y.COOH COO hydrogen bond in it.

As for the experimentally determined struc-tures, we find that the localized hydrogen bond

˚ w xdistance is 2.044 A 1 , shorter than that of L1˚ ˚Ž .2.102]2.124 A in this study at about 0.06]0.08 A,

˚and also find that its d is shorter at 0.08 AO — Hbut, bond angles are larger at about 28. Theseresults indicate that the hydrogen bonding effect

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 547

CHEN AND SHYU

TABLE III˚( ) ( )Optimized bond distances and angles in degrees related to various types of hydrogen bond in A .

Form Hydrogen bond type d d d d d / /C} C C} O C5 O O } H O } H CCO COHa ( )( )Obs. Inter- COOH COOH 1.548 1.339 1.208 1.056 2.044 111.9 104.4

b( )( )L1 Inter- COOH COOH 1.534 1.331 1.220 0.976 2.102 113.4 106.3c1.543 1.326 1.210 0.979 2.122 113.2 106.6d1.545 1.325 1.202 0.976 2.124 113.4 107.4e1.541 1.331 1.212 0.973 2.122 113.5 107.0

( )( )Inter- COOH COOH 1.535 1.343 1.224 0.974 2.062 111.2 107.01.545 1.329 1.216 0.977 2.051 110.9 107.31.546 1.337 1.208 0.975 2.075 111.2 108.01.541 1.343 1.218 0.971 2.081 111.4 107.7

( )L2 Intra- COOH 1.334 0.972 2.327 106.61.338 0.973 2.328 107.41.326 0.971 2.331 108.01.332 0.969 2.328 107.0

No H bond 1.211 124.11.201 124.21.193 124.21.204 123.9

( )L3 Intra- COOH 1.530 1.346 1.215 0.972 2.316 125.5 105.71.539 1.341 1.206 0.973 2.314 125.3 106.41.542 1.338 1.198 0.971 2.319 125.4 107.21.535 1.344 1.209 0.969 2.316 125.5 106.2

( )L4 Intra- COOH 1.530 1.348 1.214 0.972 2.314 125.5 105.71.539 1.344 1.204 0.973 2.313 125.2 106.4

f1.540 1.338 1.198 0.971 2.321 125.4 107.21.536 1.346 1.208 0.969 2.315 125.3 106.2

( )L5 Inter- OH 1.538 1.348 0.969 2.003 115.4 109.221.548 1.345 0.971 2.019 115.3 109.71.552 1.344 0.969 2.041 115.4 110.21.544 1.347 0.969 2.025 115.4 109.2

( )Intra- COOH 1.365 1.210 0.971 2.340 124.9 107.21.361 1.199 0.973 2.340 124.6 107.81.359 1.191 0.971 2.342 124.6 108.31.362 1.203 0.969 2.340 124.4 107.5

b gL6 No H bond 1.536 1.366 1.207 0.969 2.202 110.0 114.31.546 1.362 1.197 0.970 2.234 110.8 114.6

e1.541 1.364 1.201 0.966 2.239 110.5 114.4y1 y 1( )( )L1 Inter- COOH COO 1.573 1.355 1.284 1.010 1.633 109.0 96.8

1.587 1.348 1.279 1.018 1.619 108.5 97.11.591 1.349 1.284 1.014 1.696 109.2 99.21.578 1.355 1.278 0.995 1.696 109.6 98.4

No H bond 1.244 127.61.235 127.91.229 127.81.239 127.6

y1 ( )L2 Intra- COOH 1.542 1.381 1.227 0.972 2.187 118.9 102.41.556 1.377 1.221 0.975 2.190 118.8 103.11.550 1.372 1.221 0.971 2.221 119.6 104.71.546 1.377 1.224 0.969 2.193 119.0 103.0

No H bond 1.257 128.31.249 128.11.245 127.31.253 128.1

( )Continued

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CONFORMER AND INTRAMOLECULAR HYDROGEN BONDING

TABLE III( )Continued

Form Hydrogen bond type d d d d d / /C} C C } O C5 O O } H O } H CCO COHa ( )( )Obs. Inter- COOH COOH 1.548 1.339 1.208 1.056 2.044 111.9 104.4

y2L No H bond 1.556 1.279 117.11.573 1.273 117.11.569 1.266 116.91.559 1.275 116.9

aRef. 1. gas-phase electron diffraction data.bMP2 / 6-31G** data.cB3LYP / 6-31G** data.d ( )B3LYP / 6-311 + G 2d,p data.eQCISD / 6-31G** data.fMeans L4+.gMeans d .H} H

Ž .in COOH L1 structure as predicted by the opti-2mization methods in this work is a little underesti-

Ž .mated. In L5 conformer, the inter- OH -type local-2˚ized hydrogen bond distance is about 2.0 A, the

Ž .shortest distance among all the inter- COOHŽ . Ž .COOH types of COOH , but it is not the2

Ž .strongest hydrogen bond. The inter- OH hydro-2gen bond is the weakest amongst all the calculated

˚Ž .values and is shortest 2.051]2.081 A correspond-ing to d of L2. This result also matches wellO — Hwith the fact that L5 exhibits the highest localminimum among five planar optimized structures

Ž .of COOH . It also clearly evidences that the dis-2tance d is not a proper factor to determine theO — Hstrength of localized hydrogen bonding in differ-ent types of hydrogen bonds. The optimized inter-Ž .Ž y1 .COOH COO localized hydrogen bond dis-

˚Ž .tance is between d 1.619 and 1.696 A, muchO — Hshorter than the ordinary local hydrogen bond

˚Ž .distance 2.0 A . Therefore we assign the inter-Ž .Ž y1 .COOH COO -type localized hydrogen bond inL1y1 of this ion to be a very strong one.

HYDROGEN BOND ENERGY ESTIMATION

The related hydrogen bond energies may beestimated through examining the energy differ-ences of various optimized structures, if the othergeometrical effects are ignored. We assign theenergy of L1 to be the reference point, withoutfocusing on its minor nonplanar hydrogen bondeffect. There are three kinds of hydrogen bondsin the five low-energy conformers of oxalic acid:

Ž Žthe interfunctional groups OH O5COH de-.fined as DE , the weak interfunctional groups1Ž .HO ??? HOOC defined as DE , and the intra-2

Ž .COOH defined as DE . Under this assumption,3L1 is constructed with two DE , L2 is formed with1one DE and one DE , and both L3 and L4 contain1 3two DE ’s. In L5, the higher energy conformer, its3structure is established with one DE and one2

Ž .DE . Using the E L1 s 0 as the reference point as3shown in Table I, we get

w Ž . Ž .x Ž . Ž .DE s E L6 y E L1 r2 s E L6 r2 11

DE may be calculated in three ways. Because the3energy difference between L1 and L2 is considered

1Ž .as DE y DE , the first calculation of DE is1 3 3

1 Ž . Ž . Ž .DE y DE s E L2 y E L11 3

Ž .1 Ž . Ž . Ž .s E L2 DE s DE y E L2 23 1

The second and third calculations of DE are3

2 Ž . w Ž . Ž .x Ž .DE s E L6 y E L3 r2 33

3 Ž . w Ž . Ž .x Ž .DE s E L6 y E L4 r2 43

Owing to the C5O, C5O and OH, OH repulsion3Ž . 2Ž .effect, the DE is usually smaller than DE . In3 3

1Ž .L2, because the DE and DE share the unique1 31Ž .C5O oxygen electron donating effect, DE is3

2Ž . 3Ž .smaller than DE and DE . According to the3 32Ž .above-mentioned assumption, the largest DE is3

reasonably an isolated DE energy. If it is true,3DE may be calculated by2

Ž . Ž . 2 Ž . Ž .DE s E L6 y E L5 y DE 52 3

Ž . y1For the COOH COO ion

y1 y1 Ž y1 . Ž y1 . Ž y1 .DE y DE s E L2 y E L1 s E L21 3

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CHEN AND SHYU

TABLE IV( )Hydrogen bond energy kJ ///// mol .

MP2 / B3LYP / B3LYP / QCISD /a( )6-31G** 6-31G** 6-311 + G 2d,p 6-31G**

DE 30.7 31.8 31.8 29.71DE 16.5 16.4 16.9 15.921( )DE 23.0 21.7 21.5 22.732 b( )DE 26.2 24.2 23.8 25.533( )DE 25.4 23.4 23.8 24.73

y 1cDE 77.5 74.5 59.7 70.11y 1DE 26.2 24.2 23.8 25.53

aUsing B3LYP / 6-31G** L6 as reference energy.b 2( )select DE as the isolate DE for DE calculation.3 3 2c ( y 1) 2( )Is calculated by E L2 + DE .3

y1 2Ž .We assume DE s DE :3 3

y1 Ž y1 . 2 Ž . Ž .DE s E L2 q DE 61 3

All the calculated hydrogen bond energies arelisted in Table IV.

Discussion and Conclusion

In summary, some important points can be dis-cussed and concluded as follows.

1. The calculated results of molecular stabilityby four methods in this study seem verysimilar. It is confirmed that the conformer L1in the C structure with double-interfunc-2 htional-group hydrogen bonds is the most sta-ble. The non-hydrogen bonding barriers

Žamong L6 ª L1 are very small 1.9 kJrmolat MP2r6-31G**, 0.8 kJrmol at B3LYPr6-

.31G** , which indicates L6 is the most unsta-Ž .ble one. In the B3LYPr6-311 q G 2d,p

method, L6 will disappear via optimizationprocedure and automatically rotate to L1.

Ž .Because the B3LYPr6-311 q G 2d,p methodŽ .gives the lowest E , E 0 K and the largestSCF

Ž .energy gap for COOH conformers, it is2more reliable than the other three methods. Ittakes more effects on correlation and basis

wfunctions puts 2 d functions on heavy atomsŽ .plus a diffuse function and 1 p function on

xhydrogens and has smaller mean absoluteand standard deviations for most moleculesw x21 . Moreover, the diffuse function we em-ployed here for the anion system is impor-

w xtant 21 . From the Tables I and IV

Ž . y1COOH COO anion data, we think thatŽ .E , E 0 K , and hydrogen bond energiesSCF

are overestimated about 10—15 kJrmol ifcalculated by three other methods.

2. The calculated result in this study tells usthat the transition states of internal rotationbarriers in the high-energy conformers,L3—L6, are significantly lower than thoseexisting in the stable conformers, L1 and L2.Accordingly, the high-energy conformers ofŽ .COOH may not be easily observed through2various experiments.

3. Among all the optimized structures, the pla-Ž .nar-type structures of COOH and2

Ž . y1COOH COO have been found more stablerespectively than those of the nonplanar L6and L2y1. On the other hand, one may con-clude that formation of planar structures con-nects with the interfunctional groups or

Ž .intra- COOH types of hydrogen bonds.Without the hydrogen atom or hydrogenbonding effect, the optimized structure ofŽ .y2COO ion will be a D -type nonplanar2 2 done.

4. The computed values of the engergy differ-ences of various optimized structures areabout 32 kJrmol for interfunctional grouphydrogen bonding, about 17 kJrmol for weakinterfunctional group hydrogen bonding, andabout 24 kJrmol for intra-COOH hydrogenbonding if calculated by the B3LYPr6-311 qŽ .G 2d,p method.

5. From the viewpoint of the molecular energyand optimized bond distances, the intra-Ž .COOH -type localized hydrogen bond hassome weak hydrogen bonding effect. More-over, the molecular energies of the intra-Ž .COOH -type hydrogen-bonded conformers,L3 and L4, are lower than that of L5 with one

Ž .interfunctional group and one intra- COOHtype bonded. Both L3 and L4 are more stablethan the non-hydrogen-bonded L6, which

Ž .may be mainly due to the intra- COOH -typehydrogen bonding effect. From the geometri-cal point of view, both the d and dC 5 O O — Hof L3 and L4 conformers are relatively longerthan the corresponding distances of L5 andL6 conformers, which provides additionalsound evidence to prove the existence of

Ž .intra- COOH -type hydrogen bonding effect.6. In the calculation of anion-type structures,

the most noteworthy result obtained in this

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CONFORMER AND INTRAMOLECULAR HYDROGEN BONDING

work is the particularly strong hydrogenbonding effect in the L1y1 conformer ofŽ . y1COOH COO ion, which has the signifi-

˚cantly short d s 1.696 A and largeH — OŽ .BE s 59.7 kJrmol by the B3LYPr6-311H — O

Ž .q G 2d,p method.

ACKNOWLEDGMENT

The author would like to thank the NationalScience Council of the Republic of China for thefinancial support for this work under Grant No.Nsc 88-2113-m-014-002.

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