infrared matrix isolation and ab initio quantum mechanical study of dimethyl ether–methanol...

Infrared matrix isolation and ab initio quantum mechanical studyof dimethyl ether–methanol complex

Sang Woo Han, Kwan Kim*

Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, South Korea

Received 16 February 1998; revised 17 April 1998; accepted 5 May 1998

Abstract

Infrared absorption spectra of dimethyl ether (DME) and methanol mixtures were recorded in solid argon matrix at 9 K. Anumber of intramolecular complex bands were observed both in the DME and methanol fundamental regions. From theconcentration dependence of infrared spectral patterns, DME and methanol seemed to form at least two types of 1:1 binarycomplexes. Referring to the ab initio SCF, MP2, and DFT level computations, the most stable complex was concluded topossess a trans near linear H-bond, formed between the ethereal oxygen and the hydroxyl hydrogen atoms. The next stable 1:1DME–methanol complex was proposed to assume a geometry with a H-bond angle to be at ca. 1508. q 1999 Elsevier Science B.V.All rights reserved.

Keywords:Dimethyl ether Methanol; Infrared; Matrix isolation; Ab initio

1. Introduction

The ethereal group is one of numerous functionalgroups that govern the characteristics of bio-molecules. In this regard, extensive studies weremade both experimentally and theoretically to eluci-date the binding properties of ether molecules withother biologically interesting molecules such aswater [1–4], alcohols [1,5–8] and hydrogen halides[9–16]. For instance, Engdahl and Nelander [3] con-cluded from a matrix-isolation infrared spectroscopythat dimethyl ether (DME) and water should form a1:1 complex with a structure analogous to that ofwater dimer. Bakkas et al. [17] reported that the struc-ture of water–methanol complex should be similar to

that of water dimer also. This information suggeststhat the DME–methanol complex may possess astructure similar to that of water dimer.

Recently we have suggested from infrared matrixisolation experiments and ab initio calculations thatthe most stable 1:1 acetone–methanol complexshould assume a near planar six-membered ring-likestructure, implying the presence of the C–H…O con-tact interaction between the hydrogen atom of acetoneand the oxygen atom of methanol along with a muchstronger H-bond between the carbonyl oxygen and thehydroxyl hydrogen atoms [18]. Recalling that themost stable water dimer has a trans near linear struc-ture [4,19–21], the C–H…O contact interaction isthought to be very unlikely to occur in the DME–methanol complex. To check this, we have performedan infrared matrix-isolation experiment for mixturesof DME and methanol in solid argon along with an ab

Journal of Molecular Structure 475 (1999) 43–53

0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00494-3

* Corresponding author. Fax: +82-2-874-3704 and +82-2-889-1568; E-mail: [email protected]

initio quantum mechanical calculation. The primaryobject of the present investigation is to provide anadditional data on H-bonding to validate our previousproposition that the most stable 1:1 acetone–methanolcomplex should assume a near planar six-memberedring-like structure. An attempt was made to clarify thepreviously unnoticed multiplet peaks appearing in thematrix-isolated infrared spectra of H-bonded com-plexes of methanol. In addition, a possible complica-tion that might occur in interpreting the spectra ofDME–water complex caused by the presence of twohydroxyl groups in water was expected to be avoid-able in the DME–methanol system.

2. Experimental

Initially, DME (Aldrich, .99% purity), methanol-h4 (Carlo Erba, 99.9% purity), and methanol-d4

(Aldrich, 99.8% purity) were degassed by repeatedtrap-to-trap distillation at 77 K. Argon (99.999%purity) was transferred to a Pyrex bulb via a flexiblestainless bellow immersed in liquid nitrogen and con-nected to a greaseless vacuum system. The gaseoussamples were mixed in appropriate ratios (1:100–2000) using a standard manometric technique. Thegas mixtures were left overnight to attain equilibriumand then sprayed onto a cold CsI window at 9 K.Unless otherwise specified, deposition was performedfor 2 h, maintaining the deposition rate at ca.0.6 mmol h−1 using a fine metering valve. The methodof infrared spectral measurement at cryogenic tem-perature was reported previously [18].

3. Computational

Ab initio SCF (self-consistent field), MP2 (Mo¨llerand Plesset second order), and DFT (density func-tional theory) calculations were performed with theGaussian 94 program [22] running on either a Cray-C90 XMP/unicos or an IBM PC pentium/windows.A possible minimum energy structure of binaryDME–methanol complex was sought not onlywith the standard 3-21G, 6-31G**, and 6-31+G**basis sets at the SCF level but also with the 6-31G**and 6-31+G** basis sets at the MP2 and DFT levels.In the DFT calculation, Becke’s three-parameter

hybrid functional [23] with gradient corrections pro-vided by the Lee–Yang–Parr correlation functional[24], i.e. B3-LYP, was employed by recalling thatthis functional was found not only to yield reac-tion energies quite accurately for a wide range ofprocesses [20], but, also to predict the properties ofhydrogen-bonded systems pretty well [18,20,25]. Ateach level, the harmonic vibrational frequencies werealso computed. To reduce the basis set superpositionerror (BSSE) in calculating the binding energies, thecounterpoise method [26] was applied.

4. Results and discussion

4.1. Infrared spectral analysis

Initially, infrared spectra of argon matrices contain-ing only either DME or methanol were examined atthe m/a (matrix/absorbent) ratios ranging from 100 to2000. The observed spectral patterns were similar tothose of previous investigators [2,3,18,27–30]. In thespectra of DME at low m/a ratios (,100), severalweak bands resulting from aggregated species wereidentified, along with stronger bands caused by iso-lated species. The spectrum of methanol wascomparatively much more susceptible to the concen-tration as well as to the temperature [18]. At a m/aratio near 2000, the aggregated species, i.e.(CH3OH)2, were barely detected. At a m/a ratio of250, the O–H stretching peaks caused by the(CH3OH)2 species were identified clearly at 3519.1,3526.3, 3533.1, and 3540.7 cm−1. The peak positionsand their relative intensities were sustained even at them/a ratio of 1000. In addition, the spectral pattern ofthe methanol dimer was hardly affected by the tem-perature increase from 9 to 35 K. The observed peakfrequencies of DME and methanol-h4 are summarizedin Tables 1 and 2 together with their spectral assign-ments.

When samples of DME/Ar and methanol-h4/Arwere codeposited on the CsI substrate at 9 K, thespectral pattern was seen to be quite different fromthat of the composite of spectra of DME and methanol-h4. All new spectroscopic features seemed to beclosely associated with the fundamentals of eitherDME or methanol-h4. It was intriguing, however,that multiplets were seen in certain spectral regions.

44 S.W. Han, K. Kim/Journal of Molecular Structure 475 (1999) 43–53

Tab

le1

Obs

erve

dan

dca

lcul

ated

vibr

atio

nalf

requ

enci

esas

soci

ated

DM

Efu

ndam

enta

lsa

Obs

erve

dC

alcu

late

d(w

ith6-

31+G**

basi

sse

tA

ssig

nmen

td

MP

2,M

ost

stab

leB

3-LY

P,

Mos

tst

able

B3-

LYP

,H

-bon

dan

gle

=15

08

DM

ED

ME

/met

hano

l-h4b

1:1

DM

E/m

etha

nol-h

4co

mpl

exc

Isol

ated

Dim

eric

2986

.5*#

2995

.7(+9

.2)e

2994

.2(+7

.7)

2998

.5(+1

2.0)

3008

.0(+2

1.5)

n1(

A1)

CH

3as

ymst

r.#2

994.

0(+7.

5)e

2994

.4(+7

.9)

2997

.7(+1

1.2)

2990

.0(+3

.5)

n12

(B1)

CH

3as

ymst

r.28

20.6

#282

9.0(+

8.4)

2835

.6(+1

5.0)

2840

.5(+1

9.9)

2837

.3(+1

6.7)

n2(

A1)

CH

3sy

mst

r.*#

2826

.1(+5

.5)

2837

.8(+1

7.2)

2843

.7(+2

3.1)

2839

.0(+1

8.4)

n13

(B1)

CH

3sy

mst

r.14

27.4

#142

4.4(−

3.0)

1427

.9(+0

.5)

1429

.6(+2

.2)

1428

.6(+1

.2)

n15

(B1)

CH

3sy

mde

f.12

44.7

1248

.0*1

249.

6(+4.9

)12

48.9

(+4.2

)12

51.0

(+6.3

)12

51.5

(+6.8

)n

5(A

1)C

H3

rock

.12

48.6

(+3.9

)11

72.3

1168

.7*1

168.

1(−4.2

)11

68.2

(−4.1

)11

68.9

(−3.4

)11

69.0

(−3.3

)n

16(B

1)C

H3

rock

.11

65.8

(−6.5

)11

72.8

(+0.5

)11

72.3

(+0.0

)11

72.0

(−0.3

)n

20(B

2)C

H3

rock

.10

98.5

1096

.3*1

094.

3(−4.2

)10

93.1

(−5.4

)10

94.1

(−4.4

)10

96.2

(−2.3

)n

17(B

1)C

OC

asym

str.

*109

2.8(

−5.7

)10

90.7

(−7.8

)92

6.0

923.

891

7.8(−

8.2)

914.

6(−11

.4)

916.

9(−9.

1)91

8.1(−

7.9)

n6(

A1)

CO

Csy

mst

r.91

5.8(

−10.

2)*9

13.4

(−12

.6)

a Wav

enum

ber

incm

−1.V

alue

sin

pare

nthe

ses

repr

esen

tthe

freq

uenc

ysh

ifts

with

resp

ectt

oth

eis

olat

edD

ME

.b P

eaks

mar

ked

with

anas

teris

kco

rres

pond

toth

em

odes

ofD

ME

that

beco

me

iden

tified

clea

rlyw

hen

the

met

hano

l-h

4co

ncen

trat

ion

isre

lativ

ely

high

er.P

eaks

mar

ked

with

a#

sign

corr

espo

ndto

the

mod

esof

DM

Eth

atar

eid

entifi

edto

bem

uch

broa

der

than

othe

rin

tera

ctio

npe

aks.

c Mod

e-by

-mod

esc

aled

freq

uenc

ies.d M

ade

byre

ferr

ing

toR

efs.

[2,2

8].e O

bser

ved

inth

eD

ME

/met

hano

l-d4

spec

tra.

45S.W. Han, K. Kim/Journal of Molecular Structure 475 (1999) 43–53

Tab

le2

Obs

erve

dan

dca

lcul

ated

vibr

atio

nalf

requ

enci

esas

soci

ated

met

hano

l-h

4fu

ndam

enta

lsa

Obs

erve

dC

alcu

late

d(w

ith6-

31+G**

basi

sse

t)A

ssig

nmen

td

MP

2,M

ost

stab

leB

3-LY

P,

Mos

tst

able

B3-

LYP

,H

-bon

dan

gle

=15

08

Met

hano

l-h4

Met

hano

l-h4/

DM

Eb

1:1

Met

hano

l-h4

/DM

Eco

mpl

exc

Isol

ated

Dim

eric

3666

.635

40.7

*353

4.1(−1

32.5

)34

98.4

(−168

.2)

3497

.3(−1

69.3

)35

32.2

(−134

.4)

nOH

(A9)

O–

Hst

r.35

33.1

3526

.334

73.3

(−193

.3)

3519

.130

05.0

2978

.429

85.0

(−20.

0)29

83.7

(−21.

3)29

86.0

(−19.

0)nC

H3(

A9)

CH

3as

ymst

r.29

55.4

2935

.3(−2

0.1)

2932

.9(−2

2.5)

2936

.2(−1

9.2)

nCH

3(A

0)C

H3

asym

str.

2847

.528

32.3

#284

3.6(−3

.9)

2832

.8(−1

4.7)

2832

.2(−1

5.3)

2834

.1(−1

3.4)

nCH

3(A

9)C

H3

sym

str.

1474

.114

74.2

(+0.1

)14

75.2

(+1.1

)14

75.3

(+1.2

)dC

H3(

A9)

CH

3as

ymde

f.14

68.5

1466

.5(−2

.0)

1467

.2(−1

.3)

1467

.8(−0

.7)

dCH

3(A

0)C

H3

asym

def.

1451

.914

66.1

1448

.8(−3.1

)14

50.5

(−1.4

)14

50.9

(−1.0

)dC

H3(

A9)

CH

3sy

mde

f.14

64.5

1332

.013

81.0

(+49.

0)14

05.2

(+73.

2)13

93.9

(+61.

9)13

71.6

(+39.

6)dO

H(A

9)O

–H

bend

.10

76.6

1080

.2(+3

.6)

1125

.1(+4

8.5)

1119

.9(+4

3.3)

1116

.4(+3

9.8)

gC

H3(

A9)

CH

3ro

ck.

1034

.010

53.5

*104

9.6(+1

5.6)

1057

.0(+2

3.0)

1057

.6(+2

3.6)

1056

.4(+2

2.4)

nCO

(A9)

C–

Ost

r.10

38.4

1048

.0(+1

4.0)

a Wav

enum

ber

incm

−1.V

alue

sin

pare

nthe

ses

repr

esen

tthe

freq

uenc

ysh

ifts

with

resp

ectt

oth

eis

olat

ed

met

hano

l-h

4.b P

eaks

mar

ked

with

anas

teris

kco

rres

pond

toth

em

odes

ofm

etha

nol-h

4th

atbe

com

eid

entifi

edcl

early

whe

nth

eD

ME

conc

entr

atio

nis

rela

tivel

yhi

gher

.Pea

ksm

arke

dw

itha

#si

gnco

rres

pond

toth

em

odes

ofm

etha

nol-

h4th

atar

eid

entifi

edto

bem

uch

broa

der

than

othe

rin

tera

ctio

npe

aks.

c Mod

e-by

-mod

esc

aled

freq

uenc

ies.d M

ade

byre

ferr

ing

toR

efs.

[29,

30].


For instance, as marked in Fig. 1, two new peaksappeared at 3473.3 and 3534.1 cm−1 in the O–Hstretching region. One may argue that the latter peakis associated with the (CH3OH)2 species. However,we believe that the peak has nothing to do with the(CH3OH)2 species (vide infra). In addition, webelieve that the two peaks have nothing to do with

the formation of a 1:2 DME–methanol complex;even at the DME/methanol/Ar ratio of 4/1/2000where no isolated dimeric methanol was detectable,two perturbed O–H stretching peaks could be identi-fied although weak (see further discussion later).

The four peaks marked in arrows in Fig. 1(a) are infact supposed to arise mainly from the (CH3OH)2

species. Their peak positions as well as their relativeintensities are hardly different from those of an argonmatrix containing only methanol-h4. The most intensepeak is located at 3526.3 cm−1 and the nearby peak at3533.1 cm−1 is comparatively much weaker. Upon theincrease in the DME concentration, the peak at3526.3 cm−1 becomes, however, no longer the mostintense peak, as can be seen in Fig. 1(b) the mostintense peak appears at 3534.1 cm−1 and its peakposition does not match with that of the (CH3OH)2

species. The new peaks at 3473.3 and 3534.1 cm−1

in Fig. 1 grew upon annealing at 35 K. Their relativepeak intensities were varied by the thermal treatment,the peak at 3473.3 cm−1 was intensified by 1.8 timeswhile the peak at 3534.1 cm−1 was intensified by 1.2times upon increasing the temperature from 9 to 35 K.This implies that the origin of those two peaks shouldbe different.

As marked in Fig. 2, more than one new peakappeared in the several DME fundamental regions.They were intensified upon increasing the concen-tration of methanol, even at the condition where thedimeric species, i.e. (CH3OH)2 and (DME)2, washardly detectable, their presence was identified albeitweak. All those new bands grew and/or becamebroadened upon annealing at 35 K. When samplesof methane/Ar and methanol-h4/Ar were codeposited

Fig. 1. Infrared spectra in the O–H stretching region for mixturesdeposited on CsI at 9 K, at the methanol-h4/DME/Ar ratios of: (a)4/1/1000; and (b) 1/4/1000. The peak marked with a ‘*’ sign iscaused by DME. Peaks marked with arrows are mainly due to(CH3OH)2 (see text).

Table 3Relative O–H stretching frequency shifts of some O–H…O hydrogen bonding systems and proton affinities of water, methanol, DME andacetone

Proton (n0 − n)/n0a Ref. Proton affinity (kJ mol−1)b

Acceptor Donor

Water Water 0.018 [19] Water 688.8DME Water 0.026 [3] Methanol 756.0DME Methanol 0.053 This work DME 781.2Acetone Methanol 0.045 [18] Acetone 798.0Water Methanol 0.039 [17]

an0 andn correspond to the O–H stretching frequency of the free and complexed proton donor molecules, respectively.bData taken from Ref.[31].


on CsI, any multiplets such as those in Figs. 1 and 2were not present at all.

It has to be mentioned that in all our experimentalconditions, methanol trimer bands were not identified.The O–H stretching modes of methanol trimer areknown to appear in the spectral region of 3482–3505 cm−1 [29]. If a 1:2 complex, i.e. (DME)-(CH3OH)2, were formed, its O–H stretching peaksshould have red-shifted with respect to those ofmethanol trimer, since the proton affinity of DME ishigher than that of methanol (see Table 3). The multi-plets at 3473.3 and 3534.1 cm−1 in Fig. 1 are, thus,thought to have nothing to do with the formation ofsuch a 1:2 complex. All this information dictates thatthe multiplets in Figs. 1 and 2 should arise fromdifferent 1:1 intermolecular complexes of DME andmethanol.

When samples of DME and methanol-d4 werecodeposited on the cold substrate, the directions aswell as the magnitudes of peak shifts of fundamentalmodes were comparable to the case of DME/methanol-h4/Ar mixture. We could identify the CH3asymmetric stretching mode of DME to be perturbedclearly by methanol-d4. In addition, two new peakswere seen at 2569.7 and 2610.9 cm−1 in the O–Dstretching region. Correlating these O–D stretchingpeaks with the two O–H stretching peaks at 3473.3and 3534.1 cm−1 in Fig. 1, the isotopic frequency

ratios, i.e.n(OH)/n(OD), are computed to be 1.3516(=3473.3/2569.7) and 1.3536 (=3534.1/2610.9).Such different ratios reflect also that the multipletsshould occur from the presence of several different1:1 binary complexes.

Tables 1 and 2 include the positions of new peaksassociated with the DME and methanol-h4 funda-mentals (see the third column), the amount of peakshifts with respect to the isolated species are denotedin the corresponding parentheses. In fact, spectralpeak-shifts occurring in the DME/methanol-h4 mix-ture were comparable to those observed for a DME/water-h2 mixture [3] and a methanol-h4/water-h2

mixture [17]. The role of methanol as a protondonor in forming the DME–methanol complex canbe evidenced from the substantial red-shift of theO–H stretching (by 132.5 and 193.3 cm−1) as wellas the substantial blue-shifts of the O–H bending(by 49.0 cm−1) and the C–O stretching (by 14.0 and15.6 cm−1) modes of methanol moiety (see Table 2).Similar peak shifts occurred for other systems likemethanol–water [17] and methanol–acetone [18]complexes in which methanol acted as a protondonor. Previous spectral data reveal that the H-bondedinteraction between the ethereal oxygen and themethanol hydrogen atoms will be the most importantin forming the 1:1 DME–methanol complex(es). Onthe other hand, the blue shift of the CH3 stretching

Fig. 2. Infrared spectra in the DME fundamental regions for mixtures deposited on CsI at 9 K, at the DME/methanol-h4/Ar ratios of (bottom) 4/1/1000, (middle) 1/1/1000, and (top) 1/4/1000. Annotated peaks are associated with features that grow in intensity with increasing methanol-h4

concentration.


modes of the DME moiety can be explained by resum-ing that thej* orbitals at the methyl group of DMEare hyperconjugative acceptors with respect to thenearby oxygen lone pair electrons. Upon forming anH-bond, the oxygen lone pair electron density will bereduced, thereby the hyperconjugative electron donat-ing capability of the lone pair orbitals becomes low-ered, resulting in the strengthening of the C–H bonds.

4.2. Comparison of H-bond strength

The O–H…O bond strength can be related to therelative peak shift of the O–H stretching mode, i.e.(no − n)/n in which no andn correspond, respectively,to the O–H stretching frequencies of free and com-plexed proton donor molecules [32]. Table 3 illus-trates the relative O–H stretching frequency changesfor a few relevant systems. The H-bond in the DME–methanol complex is seen stronger than that in theDME–water complex. This can be attributed to themore acidic nature of methanol than water [31].Recalling the substitution effect of the methyl groupthat the proton affinity increases in the order of water, methanol, DME [1,33–35], one can understandthe H-bond in the DME–methanol complex to bestronger than that in the water–methanol complex.A comparatively higher H-bond strength of theDME–water complex over the water–water complexcan be explained similarly.

Although the proton affinity of acetone is greaterthan that of DME, the relative O–H stretching fre-quency shift is larger for the DME–methanol complexthan for the acetone–methanol complex. This may beattributed to the structural difference between the twocomplexes. As mentioned previously, the acetone–methanol complex was concluded to possess a planarsix-membered ring like structure by forming twokinds of H-bonds, one between the hydroxyl hydrogenand the carbonyl oxygen atoms and the other betweenthe oxygen atom of methanol and a hydrogen atom ofacetone [18]. Rsulting from the latter C–H…O con-tact interaction, the O–H stretching frequency shift inthe acetone–methanol complex is thought to be not somuch dramatic as that in the DME–methanol com-plex. This implies that the C–H…O contact inter-action is less important in the DME–methanolcomplex than in the acetone–methanol complex. Itis also noteworthy that the magnitude of frequency

difference between the two perturbed O–H stretchingpeaks in the DME/methanol-h4 complex (60.8 cm−1)is nearly 4 times larger than that in the acetone-h6/methanol-h4 complex (15.5 cm−1) [18]. Consideringthat the structure of acetone–methanol complex israther restricted by the presence of two kinds of H-bonds, the O–H bond strength will not be susceptibleto a structural variation between two different con-formers. For the case of DME–methanol complexthat might possess only one kind of H-bond, a com-paratively large structural distortion could be allowedso that the O–H bond strengths of two different localminimum structures were greatly different. On theother hand, recalling that the direction of H-bond isdetermined usually by the orientation of the nonbond-ing electron pairs in acceptor molecules [36], such astructural difference may be related with the fact thatthe oxygen lone pair electrons in DME are directedaway from the molecular plane while those in acetoneare located in the molecular plane.

4.3. Ab initio geometry of the most stable 1:1 DME–methanol complex

On the grounds that the 6-31G** and 6-31+G**basis sets described the H-bonded complexes veryaccurately [37], at the beginning the optimized struc-tures of DME and methanol were obtained first andthen their fundamental vibrational frequencies anddipole moments were computed with these two basissets. No structural constraint was imposed on any ofthe geometry optimization routine. The mode-by-mode scaling factors [18] were computed by compar-ing the theoretical frequencies with those observed inthe matrix isolated spectra. The mean scaling factorsand dipole moments are listed in Table 4. Uponincorporating the electron correlation effect, the com-puted frequencies became closer to the observedvalues. The dipole moments were predicted betterwith the 6-31G** basis set than with the 6-31+G**basis set. Considering that the coulombic energy isusually the dominant term in H-bonding energy[39], the former basis set may be thought to be moreappropriate than the latter in the description of the H-bonded complexes. As will be seen later, the formerbasis set seemed, however, inferior to the latter at leastfor the DME–methanol complex.

We mentioned earlier that spectral peak-shifts


occurring in the DME/water-h2 and methanol-h4/water-h2 mixtures were comparable to those observedin the DME/methanol-h4 mixture. On these grounds,referring to the structures of water–water [4,19–21],DME–water [2–4] and water–methanol [17] com-plexes, we sought the stable geometries of 1:1DME–methanol complexes. Several stable geome-tries obtained initially at the SCF/3-21G level wereused as the input data in the subsequent SCF/6-31G**level optimization. The most stable configuration,thus, obtained was used as the input data in theSCF/6-31+G** level optimization. Those SCF resultswere used in the MP2 and B3-LYP level optimization,once again, no structural constraint was imposed onany of the the geometry optimization routine. For allthe optimized structures, imaginary frequency was notyielded at all. Intriguingly, the optimized structurewas more susceptible to the basis set rather than thelevel of computation method.

Selected structural parameters calculated are pre-sented in the fourth and fifth rows of Table 4. In thestructure obtained with the 6-31+G** basis set, the H-bond angle (<O1H7O2; see Fig. 3 for the numbering ofatoms) was near 1808. In addition, the two molecularsymmetry planes were near perpendicular to eachother. In contrast, the structure obtained with the 6-31G** basis set exhibited neither linear H-bondingnor perpendicular molecular symmetry planes. None-theless, a short O1…H7 distance computed, 1.85-2.01 A, suggested a stronger H-bond to be formedbetween the ethereal oxygen and the hydroxyl hydro-gen atoms. The BSSE corrected interaction energycomputed with the 6-31+G** basis set was lower

than that computed with the 6-31G** basis set,specifically by 3–4 kJ/mol under the considerationof the electron correlation effect. This may implythat the configuration calculated with the 6-31+G**basis set is a more plausible one. In fact, such a transnear linear conformation was suggested also for thewater dimer as well as the DME–water and water–methanol complexes.

Table 4Mean mode-by-mode frequency scaling factors, dipole moments of DME and methanol, and selected structural parameters and interactionenergies of 1:1 DME–methanol complex

SCF MP2 B3-LYP

6-31G** 6-31+G** 6-31G** 6-31+G** 6-31G** 6-31+G**

Mean scaling factor 0.90 0.90 0.94 0.95 0.97 0.97Dipole moment(Debye)DME(1.30)a 1.48 1.56 1.40 1.58 1.28 1.46Methanol(1.70)a 1.85 1.99 1.77 2.00 1.67 1.91H-bond(O1

…H7) length (A)b 2.0088 2.0094 1.9056 1.8549 1.9032 1.8844H-bond angle(<O1…H7-O2)(degree)b

163.5 178.8 151.8 177.2 154.3 177.6

Interaction energy(kJ mol−1) −17.32 −18.16 −18.72 −22.79 −18.28 −21.65

aExperimental values taken from Ref. [38].bSee Fig. 3 for the numbering of atoms.

Fig. 3. Changes of net atomic charges (in e, values in parentheses)and bond lengths (in A˚ ) upon forming a 1:1 DME–methanol com-plex, computed at MP2 level with a 6-31+G** basis set.


Vibrational frequencies corresponding to the bestoptimized binary complex, calculated with the 6-31+G** basis set, are presented in Table 1 for theDME moiety and in Table 2 for the methanol moiety(see the fourth and fifth columns). The theoreticalfrequencies are the mode-by-mode scaled values,made by referring to the scaling factors for uncom-plexed species. The predicted frequencies wereslightly dependent on the level of calculation method.Nonetheless, the predicted peak-shifts were grossly inconformity with the experiment, specifically uponincorporating the electron correlation effect. Althoughnot shown in Tables 1 and 2, much better predictionseemed to be made with the 6-31+G** basis set thanwith the 6-31G** basis set. This together with thelower interaction energy computed suggest onceagain that the 1:1 DME–methanol complex shouldassume a trans near linear conformation.

Fig. 3 illustrates the structural changes computed atthe MP2/6-31+G** level. For the DME moiety, theC–O bonds are lengthened considerably whereas theC–H bonds are shortened. These can be related,respectively, with the red-shift of the COC stretchingmode and the blue-shift of the CH3 stretching mode.Regarding the methanol moiety, the O–H bond islengthened substantially while the C–O bond isshortened. The C–H bond is computed to be slightlyincreased. These seem to be reflected in the red-shiftsof the O–H and CH3 stretching modes as well asthe blue-shift of the C–O stretching mode. Result-ing from the stronger H-bonding, the O–H bendingmode is supposed to be significantly blue-shifted.The net atomic charges, obtained by the naturalpopulation analysis [40], are also seen to change sub-stantially, in particular for the atoms participating inthe H-bonding.

4.4. Other possible 1:1 DME–methanol complex

In order to explain the occurrence of more than oneinteraction peak, especially two O–H stretching peaks(see Fig. 1), we have assumed earlier the presence ofat least two different 1:1 complexes. In this respect, abinitio calculations were performed, at the beginning atthe SCF/6-31+G** level, to see how the bindingenergy and the vibrational frequencies are to changeas a function of H-bond angle (/O1H7O2), H-bondlength and dihedral angle; no structural constraint was

imposed on the geometry optimization routine exceptthose three parameters.

At the SCF/6-31+G** level, the H-bond angle ofthe most stable structure is 178.88 and the interactionenergy is−18.16 kJ mol−1 (see Table 4). At otherangles (150, 160 and 1708), the interaction energieswere, of course, higher than that of the most stablestructure. Nonetheless, those complexes seemed alsoto correspond to minimum structures because anyimaginary frequency was not seen at all. Their calcu-lated vibrational frequencies were slightly differentfrom those of the most stable structure. It is worthnoting that the O–H stretching frequencies computedat <O1H7O2 = 1508 was 12.8 cm−1 different from thatof the most stable structure even though the peak-shifts in other spectral regions were quite small.

At the SCF/6-31+G** level, the interaction energyincreased also as the H-bond length, i.e. distancebetween O1 and H7 atoms, and the dihedral anglebetween the two molecular symmetry planes werevaried from those of optimum structure, i.e.2.0094 A and 90.18. Once again, no imaginaryfrequency was seen at all. As the dihedral angle wasdecreased, all the vibrational frequencies weregradually deviated from those of the optimum struc-ture. As the H-bond length was decreased, the O–Hstretching mode down-shifted from that of the opti-mum structure. At 1.7 A˚ , the peak shift in the O–Hstretching mode was calculated to be−224.3 cm−1,quite close to the experimental value. However, theamount of shift in other modes was substantially dif-ferent from the experimental values. As the H-bondlength was increased, the O–H stretching mode wasup-shifted from that of the optimized structure. At2.15 A, the calculated shift in the O–H stretchingmode was different by 25.8 cm−1 from that of theoptimum structure. In this case, the peak shifts inother spectral regions were, however, rather insig-nificant. The corresponding structure may, thus, beregarded as another possible structure related to theoccurrence of two O–H stretching peaks.

Referring to the previous SCF result, we haverepeated the ab initio frequency calculation at theB3-LYP/6-31+G** level for the two cases, one withthe hydrogen bond angle to be 1508 and the other withthe hydrogen bond length to be 2.15 A˚ (the MP2 levelcalculation was not performed since similar resultswere expected). As can be noticed from the fifth and


sixth columns of Table 2, the O–H stretching peak-shifts calculated for two kinds of 1:1 complexes, onecorresponding to the most stable geometry(−169.3 cm−1) and the other corresponding to a geo-metry having a hydrogen bond angle at 1508(−134.3 cm−1), matched reasonably well with theexperimentally observed values (−193.3 and−132.5 cm−1). A similar close match was seen alsofor the O–D stretching mode. The amount of peak-shift calculated from a geometry with a hydrogenbond length of 2.15 A˚ was, however, far less conso-nant with the experimental value.

The frequencies of a few other intramolecularvibrational bands calculated for the 1:1 complexwith a hydrogen bond angle at 1508 are listed in thesixth columns of Tables 1 and 2. The peaks markedwith an asterisk in the third columns correspond to themodes of DME or methanol-h4 that become identifiedclearly when the methanol-h4 or DME concentrationis relatively higher. Although not quantitative, thesepeaks seem to be correlated with those calculated for abent complex (i.e. hydrogen bond angle= 1508).Broad bands marked with a # sign in the third columnsof Tables 1 and 2 are supposed to occur from theoverlapping of very closely spaced peaks caused bydifferent 1:1 complexes. In addition, considering thatthe O–H stretching band is intensified considerablyby forming a H-bond, it may not be unrealistic toobserve the O–H stretching peak of somewhat lessstable complex although other peaks do not appeardistinctly near the main perturbed fundamentals.

5. Conclusion

We have recorded the infrared spectra of DME/methanol/Ar mixtures at 9 K to see the possibility ofintermolecular complex formation. From the peakshifts of the intramolecular fundamental modes, theformation of the H-bonded 1:1 DME–methanol com-plex could be evidenced. The observed peak shiftswere grossly in conformity with the ab initio cal-culated ones. It appeared that at least two differentconformers coexisted in the argon medium. Nonethe-less, the most stable 1:1 complex seemed to assume atrans near linear structure. Besides, from this study,we could be more confident that the most stable 1:1acetone–methanol complex should possess a near

planar six-membered ring-like structure, implyingthe presence of the C–H…O contact interactionbetween the hydrogen atom of acetone and the oxygenatom of methanol along with a much stronger H-bondbetween the carbonyl oxygen and the hydroxylhydrogen atoms.

Acknowledgements

This work was supported in part by Seoul NationalUniversity through the S.N.U. Research Fund (1997),by the Ministry of Education through the BasicScience Research Fund (1997), and by Korea Scienceand Engineering Foundation through the Center forMolecular Catalysis at Seoul National University(1997). S. W. Han acknowledges the System Engi-neering Research Institute for allocating the time touse the Cray-C90 computer.

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infrared matrix isolation and ab initio quantum mechanical study of dimethyl ether–methanol...

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