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The structures and vibrational spectra of trimethoxymethane and tetramethoxymethane
Spectrochlmica Acts, 1967,Vol. %A, pp. 847 to 885. Permmm Pme.m Ltd. printedin NorthernIdand
The structnres and vibrational spectra of trimethoxymethane and tetramethoxymethane H. LEE* and J. K. WILMSHURST Division of Chemkal Physics, C.S.I.R.O., Chemical Rese8mh Laboratories, Melbourne, h8trali8 (Received7 Apit
1960)
Al&r&--The infrared spectra of trimethoxymethane asd tetramethoxymethanein the vapour phase, 8s solutions in CS, end Ccl, and in the solid state have been obtained from 3-26 p together with the Reman spectra of their liquids. In solution or the vapour phase, trimethoxymeth8ne exists 8s a mixture of two isomers TBB, TBa’, the l8tter being stable by 610 cal/mole, while in the solid state the molecule assumes the An assignment has been m8de for ectchspecies. QCfCfconformation having point symmetry c, Tetrsmethoxymethsne hes S, symmetry in the solid and a distorted S, symmetry in solution and the observed spectra have been assigned on this b&sis. It is suggested that the principal effect in determining the stable conform&ions of the methoxymethanes, besides the steric hindrance of the methyl groups, involves the oxygen lonepair interactions.
ALTHOUQH the i&a-red spectrum of trimethoxymethane (trimethyl-orthoformate) has been reported [l], its Reman spectrum and the infrared and Raman spectra of tetramethoxymethane (tetramethyl orthocarbonate) have not been investigated previously. In both cases no structural determinations have been made although the thio analogue of tetramethoxymethane is believed to have a configuration having point symmetry S, in the solid [2]. In the present investigation the Raman spectra of the pure liquids and the infrared spectra of the solid, solution and vapour phases of the two esters have been obtained. The data for trimethoxymethane is consistent with the co-existence of two isomers in the liquid and vapour phases, both isomers having no symmetry, and the presence of a third isomer having point symmetry C, in the solid. By contrast, the data for tetramethoxymethane is consistent with the assumption of only a single isomer persisting in all three phases and having point symmetry 84 in the solid and a distorted S, symmetry in the liquid and vapour. The observed low energy conformations for both the dimethoxy- [3] and trimethoxymethanes can be predicted on the basis of interactions between lone-pair electrons on the oxygen atoms and represent an excellent example of the important contributions lone-pair interactions may make to the stable conformations of molecules. * Present address: Department of Chemistry, J&n
Sultan, Petabng Jaya, Malaysia.
[l] K. NWADA, J. Chena. Sot. Jupan 81, 1028 (1960). [2] W. G. PERDOK end P. TERPSTRA,Rec. Traw.C&n. I, [3] J. K. WILMSEURST, Cam.J. Chem. 86, 286 (1968).
347
687 (1943).
343
H. LEE and J. K. WILMSHURST
EXPERIMENTAL The trimethoxymethane was prepared from chloroform and sodium methoxide following the method outlined in Organic Synthesis [4] whilst the tetramethoxy derivative was synthesized by the procedure of TIECKELMANN and POST [5] using sodium methoxide and thiocarbonyl tetrachloride. The infra-red spectra of the vapours and solutions in carbon disulphide (1400-400 cm-l) and carbon tetrachloride (4000-1400 cm-l) were recorded using either a Beckman IR-7 or IR-9 spectrometer whilst the solid spectra were obtained with a Perkin-Elmer model 337 instrument. The solid and solution spectra are shown in Figs. 1 and 7 for trimethoxymethane and tetramethoxymethane respectively while the vapour spectra are presented in Figs. 4 and 6 respectively. The Raman spectra of the pure liquids were recorded photographically using a Hilger source and two prism glass spectrograph having 21 &mm dispersion at 4500 8. Mercury 4358 A radiation was used for excitation, the 4047 A radiation being filtered out by a saturated solution of sodium nitrate. Depolarization data were measured by the technique of EDSALL and WILSON [6] and corrected for convergence error by the method of RANK and KAIURISE [7]. The observed spectra and depolarization ratios observed for trimethoxy and tetramethoxymethane are tabulated in Tables 1 and 3 respectively, together with the observed infia-red data for the liquids and vapours.
Trimethoxymethane The possibility of rotational isomerism, giving rise to trans and gauche isomers (Fig. 2) about each of the three C-OMe bonds in turn, allows in theory twentyseven conformations for trimethoxymethane of which fortunately only seven,* the TTT, TTG, TGG, TGG’, TG’G, GGG and GGG’, are spectroscopically distinct. In addition steric interactions between the methyl groups in four of the conformations, TTT, TTG, CCC’ and TG’G, (Fig. 3) eliminate the need to consider them seriously as likely low energy isomers and we may accordingly confine our attention to the remaining three isomers only. The GGG conformation has point symmetry C, and hence the forty-five fundamental vibrations should divide into symmetry species as F,, all vibrations
= 15~ + 15e
being active in both the infra-red
and the Raman,
* The tlransor gaucheconfigurationabout any C-OMe hydrogen &tom is denoted T or B respectively.
with the type a
bond with respect to the methae
[4] H. GSMAN and A. H. Brxrr (Editors), “Organic Synthmb” Collective Vol. 1 (2nd Ed.), p. 258, John Wiley, New York (1961). [6] H. TIECKELMANN and H. W. POST,J. Org. Chem. 13, 265 (1948). [S] J. T. EDSALLand E. B. WILSON,J. Chern. Phys. 6, 124 (1938). [7] D. H. RAN-Kand R. KUARISE, J. Opt. Sot. Am. 40, 89 (1950).
349
The structures of trimethoxymethane and tetramethoxymethme Table 1. The infra-red and Raman spectra of trimethoxymethane Infra-red spectrum Solution (cm-l)
Int.
483
w
497
m
540
W-Ill
553
w-m
Vapour (cm-‘)
495 600 606
1
640 660 6671
Raman spectrum
Band type
A
C( TBB’)
cm-l
Relative inteneity -
912
8
929 -
8
466 983
1018 -
lOS0
-912 918 -928 1 936 -944 I
“8
8
8
1071
vo
1102
“8
1126
“8
1027 1033 1 1039 1045 10511 -1079 Ml084 ~1088 1092I 1110 1114 I
V1,) - v1, = VI1 - &,) = -%Ob
200 229
203 234 309
1 0 2
0.06
347
4
0.06
413 422 436 419
0 0 0 10
0.12
%. - Vl70 = VlS
494 -
6
0.19
yls
616 539
0.21
%b
563
0.21
%P
663
0.21 UO
-V280
VO66- %a = VP66- VWb=
NO
40
-ho
VI2 vg -vu vpar,-v,,. v230- h,b VP20-ho ;‘a~; - hb
683 696 742
11 6 1
766
3
0.64
871
0
C( T&Y)
911
13
0.08
VI1
C(TW)
927 -
16
0.08
f&
= = =
606 627 666 661
=
669
=
yII?L
VZ?. Vzsa --
x
v200 =
ha
-%Ob
12
0.16
loa
3
0
1070
6
0.14
1072
8
0.14
1106
7
0.16
%.b’ Yp6b
1126
6
0.29
%.a*Va.5. -
1016
619
%VO
2 AC
=
409 422 430
V28S
0.05 0.1 dp?
w -988 991 1002I
139 186
0
602 626 VW VW VW w
varo - %3a = vBIo - v*,~ =
186
694
I367 662 672 687
Assignment
Depol. ratio
VOS. -
=
878
966
H. LEE and J. K. WIUMZORST
360 Table
I (coti.) Infra-red speotrum
Solution (our1)
I&.
vapour (om-1)
Band type
1137 A
B
1160 1143 1 -1184
-1169
m
~1190
m
-1177 I -1171 ~11881 1194 1200 ~1204
1167
Ramen speatnun cm-1
1158
Relative intensity
10
Assignment
Depol. ratio
0.27
A
B
1190
7
0.08
-
1196 -1202 1226 lRs4
1852 ~1557 1388 -
-1434 1446
* m 8 m
8 m s
1208 I
1218 1229 ~1237 I
A?
C( TGW’)
1366 1363 -1369 ~1376 1
B
~1366 1366 ~1376
U( TtX.7)
1281
0.09
1261 1304 1322
NO
%Sb
1859
6
0.5
1367
6
0.6
1377
4
0.6
m *
-1444 -1447 1463 1 1460 -
VS veab+ vdsr= 1292 y,. + y, = 1327 -_
-VPld -VW
Vpsa+ v*aa= 1449 -B
%lbt
-hlb
A 1462
20
0.72
-
1466 ~1634 ~1641 1661 1676 1606 1703 1736 1767
B w w w w w w m w
1466 ~1466 I ~1466 1476 ~1486 I
C( TUB)
C(TBff’)
V6
1473 1641
0 0
0.69
%a*
yloIv2
Km.
+
VSBb = -
F
+
y1le0
1o
5 vl,, VI1 ;:
+vls
= =
+ %E. = + v; = fv, = $ y = IBa=
1534 1649
1664 1672 1623 1711 1736 1763
The structurea of trimethoxymetbane and tetmmethoxymethane Table
361
1 (cot&) Infia-red fql‘xtrum
Solution fcm-’
Int.
Raman spectrum
Baud
VCbpOUr lorr’l
tYlJe
am-l
Relative intetitv
Depol. ratio
1773 N1796 NM03
vmb
3000 ~3542 3560 ~3606 3648 3692 3802 3905 3937
1798 1812
VSlb + VIBb= vII +v,,~ = vI1 + vps,,= vI1 +vub =
vll
2108
+ va,,, =
+ vII.
=
,,,+G= 2197 v**. + v*e = 2199 Va3) + VIOL= 2220 vz3,,+ vIab = 2226 vzsL + vpw = 2298 vaJo+ vBso = 2322
~~~~=““S” 4&b= 2668 vIlb + v, = 2603 VW, + VBab= 2635
V,
+v,
V,
+
ye
=
2686 2734
%b
+
VPtb
=
2797
=
2890
-
2847 2854 ~2870 2879 ~2888
C( TOG’) 2842 2838I C( TOO)
2903 2913
2947
=
1822 1853 1892 1942 1983 2014
vg
2782 2834
2937
v%,~= 1789
ylo + v~,,, = -vIso + va,,, =
~2176 N2199 ~2208 2217 2299 2319 2336 ~2381 2422 ~2442 N2SSS 2600 ~2637 ~2647 2667 ~2686 2730
2883
+
vsD + v,,~
1811 1863 1886 1942 1979 2001 ~2033 2066 2109 ~2127 ~2137
28371
A&gnment
~2918 2929 2940 2956 2963 -2975
20
0.05
V@
3011 -
0
3011 3016 3018 ~3021 I
B
v4 -
2874
7
0.2
2
2909
7
0.2
2 XV6 2 xv,
= 2904 = 2934
2 x Vu
= 2946
O( TOW)
C( TOO’)
=
2949
16
0.1
3005
10
0.45
x vID
-
Vu VlW v1r -- Vl. 3640 3663 111 + s& = 3699
Vl
+v**b
Vl
+
=
vaea =
Va
+vnr.
=
3646
Vl
+ va,,
=
3696
II_
+
Vl
+v,=3912
vIIb =
3820 ^___
352
H.
Table
2. The
and J. K.
WILMSJXURST
approximate description and observed frequencies of the fundamental of the three conformations of trimethoxymethane
Species Cl
LEE
C*
Number
Approximatte description of vibration CH, extsrmal
CH, internal
Observed fundamental frequencies BBP
(2976) (2967) 2843 1473 1460 -1446 1237 1139 1106 926 697 601
V14 y15
0
vibrations
TU@V
TGO’b
(3016) (3011) (2963) (2847) (1473d) (1469) 1466 1261d (1143) 10706 936 5946 600
3016 3011 2963 2847 1476 1462d 14626 1229 1143 1086 918 660 4796
(3016) (3011)
3016 3011
(2963)
2963
(14736)
14736
(1462d) (1450) (1447) 1377d 1365 1226’ -1202’ (-1171) (1143) (1126) (1114) 1033 991 756 742 3476 309d
1462* 1450 1447 1367‘ 1366 1208 1197 ml171 1143 1126 1114 1086 1042 696d 687* 663d 5408
torsionc-0 (3001) 2978 ( 2974 2960 { 2964 1484 { 1479 1452 ( 1449 1380 ( 1374 1206 ( 1200 1169 ( 1163 1090 ( 1083 1008 1 1004 601 ( 594 648 <\ 643
a I&m-red frequencies for the solid. * Infra-red frequencies for the vepour unless indicated otherwise. 0 Fmquenoiea in parenthesis have else been assigned to fundamentala of the TM d Raman data for the liquid. * Liquid data.
isomer.
The structures of trimethoxymethane Table 3. The infrared and Raman
Int.
vapour (cm-‘)
457 465 470
462
SO0 577 684 690
581
of tetramethoxymethane
spectra
Raman spectrum
Jnfra-red spectrum Solution (cm-‘)
353
and tetramethoxymethme
655 662 675 694 729 755 793 99s 1017 1037 ~1072
(cm-‘)
Relative intensity
Assignment
Depol. ratio
111 140 165 199 301 318(P) 330 403
0
P ;Ipt
459
6
0.04
V41
47s 603 625
4
0.04
VI1
;
683
5
612
0
663 675 696
! 8
P? dp P
762 773
16 4
0.01 P
P
vao
P 053 dp
P 0.52
P VlO - v,~ = %s - VI1 = V7 - vtob= %a - v, = VlZ vam - %s =
VP7
V98 -
1110
t 6 2 6
1114
3
0.46
1161
3
0.54
1196
2
0.1
1265
0
P
1346 1378
0 1
P
1452
10
0.59
OT74
VbO =
169 200 302 320 401
529
VP0 V90a - vll = 608 2 x VI* = 660 V0 - vl* = 662 ?A6 %I - vp, = 693 %8 - VI1 = 7373
VlO VOS VI9
992 1016 1035 1076 1093
108
-vll=
+
v41=
792
VP %4
of)13
1124
1167
1214 1218 1224
1212 1258 133s
%l vl,, VI*
+vdo= -+ VI1 =
1336 1346
1442 ~1462 1466 1474 1655 1602 1610 1619 1676 1701 10
w w w w
a
0.41 l& +VI1 + vBa i_ v8, f vge + vg +
vi1 = v** = vile= v,~ = vao = vz, =
1668 1599 1618 1629 1691 1699
364
H. LEE and J. K.
WILMSHURST
Table 3 (cont.) Infra-red spectrum Solution (cm-~) -1769 1783 1864 -1876 NlOOS 1966 2049 2070 2087 -2109 2123 -2202 2236 2328 2386 2411 2426 2461 2660 2601 2665
1nt.
Raman spectrum v&pour (cm-~)
Relative intensity
(cm-‘)
Depol. ratio
w
V80)+ v*s = vga +vao= vsga+ v,, = v, + vg, = VI0 + vg, = % + vaa= v, + vpg= 2 x v,, = vzl + vssb = lag + v*go= “y ; r,, _=
1761 1793 1866 1878 1922 1060 2953 2074 2093 2110 ;2;7
va8 + v”,; : 2234 vs8 + vQ8= 2336 V, + ye4 = 2396 v, + vzJ = 2416 2 X vaB = 2424 v, + vQ8~ = 2464 v, + vs = 2614 vss + vQ, = 2609 vQ4 + v%,, = 2678 2844 2860 2866
2844
16
2877
3
P
2914
2919
6
0.06
2941
2961
20
0.02
2976
2982
4
0.76
2997
3006
8
0.44
2840 2884
-3070 3309 3412 3466 3666
Assignment
0.01
2
X vs6 =
21384
V,
+ va6 =
2894
X vsb =
2932
2
Vl, V16. %I
P vg,, + vlB =
w w
1~11 + val = vIo + vll = V~O + v&l =
w w
w
3327 3422 3472 3678
modes being polarized in the latter. Calculation* of the GERHARD and DENNISON [8] parameter gives /I = -0.40 and hence the infra-red vapour spectrum should consist of two types of bands, parallel bands having a medium-strong Q branch and a PR spacing of ~18 cm-l and perpendicular type bands having a strong Q branch and PR spacing of -13 cm-l. * The moments of inertia used in calculating the GEREARD and DENNISON or BADGER and ZVMWALT pwameters were calculated (assuming rCH = 1.09 A, Q-~ = 1.44 A and all angles tetrahedral) to be: HC(OCH,),
Btw TBff ( TBCi
C(OCH,).kz.
I*i 339.13 279.26 236.55
IV? 339.13 364.34 419.94
1st 501.67 523.51 510.18
514.36 463.09
626.61 453.09
626.51 744.32
t x 1040g-cms. [S] S. L. GEREARD and D. M. DENNISON,
Whys.
Rev. 43, 197 (1933).
The structures of trimethoxymethane and tetramethoxymethane
Fig. 1. The i&a-red spectrum of trimethoxymethane. Solutions in Ccl, (3900-1400) and CS, (1400-400). Unless otherwiseindicated the concentration is 1: 10 by volume and the cell length is as shown. (a): concentration = 1: 50 by volume. (B) Glass pbtained by rapid freezing from the vapor onto a plate cooled to liquid air temperature. (C) Solid obtained by crystallization of the glassy material at ~-55% with subsequent cooling to liquid air temperature.
(4
355
H. LEE and J. K.
356
WILMSHURST
6
tram
C”3
‘V,]”
,p
CH30y’
// . \
/ ‘-OCH3
/ ’
‘\
.
CHpu’
.LOCH3
: 1
gaucho Fig. 2. The
tramsand gauche conformations about SLC-O
bond in trimethoxymethane.
The TGG conformation has no symmetry and hence all forty-five fundamental vibrations should appear in both the infra-red and the Raman, all Raman bands being polarized. Calculation of the BADGER and ZUMWALT [9] parameters give S = 0.03 and p = 0.61 and hence the infra-red spectrum of the vapour should exhibit three types of band contours, type A having a medium Q branch and PR spacing of ~13 cm-l, type B having a PQQ’R type structure with a QQ’ separation of ~4.5 cm-l and type C having a very strong Q branch and a PR spacing of -19 cm-r. Although the TGG’ conformation should ideally have one plane of symmetry and therefore give rise to twenty-five polarized and twenty depolarized bands in the Raman effect, the presence of two pairs of directly opposed lone-pairs on the oxygen atoms involved in the GG’ configuration (Fig. 3) will almost certainly cause the two OCH, groups to twist away in opposite senses from each other thereby removing this symmetry plane and causing all vibrations to be polarized in the Raman effect. The BADGER and ZUMWALT [9] parameters for the TGG’ conformation are S = -0.63 and p = O-96 and hence the infra-red spectrum of the vapour should exhibit three types of band contours, type A having a P&R structure with a medium to strong Q branch and PR separation of ~13 cm-l, [9] R. M. BADGER
and L. R. ZUNWAET, J. Chem. Whys. 6, 711 (1938).
The struotures of trimethoxymethane
and tetramethoxymethane
B
A X
-4
Y
CH3
*/-
X=H Y=OC)5 I
dG
X=0 11 Y=H “‘3
.
Fig. 3. Conformations of trimethoxymethane (A) Illustrating the steric hindrance in the G’G and TT conformations. (B)JIllustrating the lone-pair interactions involved in the GG’ conformation. (C)Jirhe GGG conformation, point symmetry C,. The methane hydrogen atom protrudes upwards from the plane of the paper. (D) The GGT conformation.
Fig. 4. The infra-red spectrum of trimethoxymethane
vapour.
367
358
H. LEE and J. K.
WIIZSHURST
type B having a PR or possibly a PQQ’R structure with a separation of -9 cm-l (P R) or possibly ~5 cm-l (QQ’) and type C having a P&R structure with a medium Q branch and PR separation of ~18 cm-l. A comparison of the i&a-red spectrum of the solutions with that of the glassy solid obtained by rapid deposition from the vapour onto a plate cooled to liquid air temperature (Fig. 1) reveals a number of sets of bands displaying differences in relative intensities between the two phases and immediately suggests the presence of two rotational isomers. This is further substantiated by the behaviour with temperature of the relative intensities of pairs of bands, notably the pair at 912, 929 cm-l, in both the infra-red and Raman, characteristic of the co-existence of rotational isomers. With these critiques the bands in both the infra-red and Raman that can be definitely assigned to the stable and less stable isomers are given in Table 1 in bold characters and underlined respectively. The Raman spectrum of the liquid contains forty-five bands of which only one is depolarized, and that doubtfully, consistent with the presence of the unsymmetrical isomers TGG and TGG’. The infra-red vapour band contours of these isomers are virtually identical except for the type C bands which exhibit a strong Q branch in the case of the TGG isomer and only a medium strength Q branch in the case of the TGG’ isomer. A study of the vapour spectrum (Fig. 4) shows that when a band known to belong to the stable isomer in the liquid gives rise to a type C contour, the contour exhibits only a medium intensity Q branch whereas when a band known to belong to the less stable isomer in the liquid gives rise to a type C contour it exhibits a strong Q branch. Thus we may conclude that the stable isomer in the liquid has the TGG’ conformation. The variation in relative intensity with temperature of the pair of bands at 912, 929 cm-l then permits us to calculate that in the liquid state the TGG isomer is less stable* by 610 cal/mole. Crystallization of the glassy material gave rise to a solid having an i&a-red spectrum different from that of either isomer present in the glass or in the liquid, the most characteristic feature of which was the presence of a number of very sharp doublets (Fig. 1). This feature is typical of that expected of a molecule possessing a three-fold or higher rotation axis sitting on a crystal site of lower symmetry, the effect of which is to remove degeneracies causing each pair of vibrations that would normally be degenerate to appear as a very close doublet and we accordingly assume that the molecular species in the crystalline state has the CCC conformation. As a preliminary to the assignment of the observed spectral data it is convenient to classify the vibrational modes of trimethoxymethane into their approximate descriptions and in particular as to whether they are internal or external modes of the methyl groups as shown in Table 2. The CCC species. This species has point symmetry C, and, if it is assumed, as above, that the crystal site symmetry effectively removes the degeneracy of the type e vibrations causing each to appear as a closely spaced doublet, then we may immediately assign each such doublet to an e type mode. Thus we assign the doublets at 2976, 2957, 1482, 1451, 1377, 1202, 1166, 1087, 1006,597 and 546 cm-l * Infra-red studies give the value 622 c&l/mole whilst Raman studies give the value 603 cal/ mole.
The structures of trimethoxymethane and tetramethoxymethaaa
360
tov 17~ v 18, v19, v21? v221 v23, v24~ v25~ v28? v21 and V~ respectively and the remaining singlet bands at 3001, 2843, 1473, 1460, 1445, 1237, 1139, 1106, 926, 597 and 501cm-ltov,,v This assignment 4, v59 v6, v79 v89 vSt v107 v117 v12 and v13respectively. is given in Table 2. The TGG and TGG’ isomers. With the aid of models and the known inertial
l-
Y
H
2 0
A
Fig. 5. Projections of trimethoxymethane on the planes containing the zz and yz inertial axes. A = TQB isomer; B = TBB’ isomer.
axis of these conformers (Fig. 6) it is possible to roughly estimate the band contours for the different vibrational modes of each isomer and the assignment of the fundamentals given in Table 2 has been made on this basis. This assignment is necessarily somewhat arbitrary and accordingly it will not be discussed further. It does however lead to a satisfactory assignment of all the remaining observed bands as overtones or combination tones (Table 1).
H. LEE and J. K. WILMSHURST
360
Tetramethoxymethane If the methane hydrogen atom in trimethoxymethane is replaced by a methoxy group then only two of the seven configurations, the TGG' and TGG, will give rise to possible low energy conformations. Ideally these should have point symmetries D,, and S, respectively but in both cases the lone-pair-lone-pair interactions of neighbouring oxygen atoms reduces these to S4 and C, respectively. For the conformation with point symmetry S, (distorted D,,) the fifty-seven fundamental vibrations should divide into symmetry species as l?,, = 14a(R, p) + 15b(I;
R, dp) + 14e(I;
R, dp)
where R, I, p and dp refer to Raman active, infra-red active, polarized and depolarized respectively. Calculation of the GERH.ARDand DENNISON [8] parameter gives @ = -0.39 and hence the infra-red vapour spectrum should consist of two types of bands, parallel bands having a strong Q branch and PR spacing of ~15 cm-l and perpendicular type bands having a medium Q branch and PR separation of ~12 cm-r. For the conformation having point symmetry C, (distorted S,), all vibrations should be active in the infra-red and polarized in the Raman. The GERHARD and DENNISON parameter [8] is calculated to be /l = 0.22 and hence all vibrations should exhibit the same infra-red band contour having a medium-strong Q branch and a PR spacing of ~12-13 cm-l. The observed vapour spectrum (Fig. 6) supports the assumption of a distorted S, structure for the stable conformation of tetramethoxymethane in the vapour while the fact that the Raman spectrum of the liquid contains thirty-nine bands of which only three are depolarized, many of the polarized Raman bands having counterparts in the infra-red, supports the assumption of the same conformation in the liquid. The infra-red spectrum of the solid is similar to that of the liquid (Fig. 7) with the notable distinction that the band at 995 cm-l in the liquid is absent and we accordingly conclude that the molecule has strictly S, symmetry in the crystalline state. In addition nearly every band appears to consist of two components separated by ~1-4 cm-l indicative of at least two molecules per unit cell unrelated by a centre of symmetry. * The assignment of the observed spectra of vapour, liquid and solid tetramethoxymethane is given in Table 3, while the fundamentals are also presented in Table 4 together with their approximate descriptions in terms of internal or external vibrations of the methyl groups or skeletal vibrations. DISCUSSION The foregoing determination of the stable conformations of both tri- and tetramethoxymethane has been based on two important assumptions, firstly, that the rotational isomerism about a carbon-oxygen bond is directly analogous to that about a carbon-carbon bond and gives rise to trans and gauche configurations (Fig. 2) and, secondly, that the' primary interaction affecting the stability of any particular configuration is the steric elect involving the methyl groups, i.e. eclipsed * This feature would beconsistent with a crystal structure fortetramethoxymethane to that (P42,,) observed for tetrathiomethyhnethae [2].
identical
The structures of trimethoxymethane and tetramethoxymethane
361
Table 4. Approximate description and observed frequencies of the fundamental vibrations of tetramethoxymethane pecies
Approximate description of vibration
Number
S4
CH, external
r v1 *2
Solid
mE =ym
-
*a
*4 *4
*0
a < *7 *0
*o
VI1 *la
6000
*11 .
VO-OJI~
*o-o 8000
*lo
VI‘
toraionc-0
BOH* BOHS
*40
so00
v416000 v44 lv43 torsiono-0
bless otherwise indicated, infrared mum data for the liquid.
Fig.
6.
Observed frequency
CH, internal
toraionoH,_o vapour data is reported.
Liquid/Vapou+ 3006 2972 2968
1474c 1473 1462” 1378O l19ec 99d 766b 476O 33oc -
3009 2986 2966 1474 1448 1464 1260 1118 1033 1019 679 791 -
(3006) (2972) (2968) (1474)C (1462)b (1473) 126Eb 1141 Io37b 1o17b 676b 773c 600 -
3004 (2986) 2966 1482 1468 1441 1212 1168 1106 1096
(3006) (2972) (2968) (1474)C 1473 1460 1218 1169 1133 10930 { 10760 684 466 -
690 470 -
b Infra-red data for the liquid.
cm -I The ix&a-red spectrum of tetramethoxymethane vapour.
H. LEE and J. K. WILM~HURST
302
Table 6. Summary of the i&era&ions Conformation
Molecule
between substituents, including lone-pairs, on adjacent oxygen atoms Interactions between adjacent oxygens
1
TT TB GW
2(1-l)” + 2(1*1)” (1.1)” f (l.l)OL + (l.Me)” + (l-Me)a 2(1*Me)O + (I*l)cI + (MeMe)=
L
BGW GQT BB’T
3(1-l)” -I- 3(1.1)” + 3(1*Me)” + 3(1.Me)LL 2(1+1)” + 3(1*1)” + 4(1*Me)” f 2(1-Me)a + (Me-Me)O: 2(1-l)’ -I- 4(1.1)” + 4(1*Me)” + 2(Me-Me)a
CH,WH,),
CWOCH,),
4(1*1)” -I- 8(l*l)c( + S(l-Me)’ + 4(Me*Me)= % 4(1*1)” + 6(l.l)a + 8(1*Me)” + 3(1-Me)O: + 3(Me*Me)OL ( s4 The supersoripts distinguish substituents directly opposed (“), i.e. lying in the same plane &B the oxygen atoms, or staggered (“), i.e. the planes oontGning the substituents and the oxygen atoms subtend an angle of -120” where they intersect on the join between the pair of oxygen WC&&
atoms.
cm-’
Fig. 7. The ix&a-red spectrum of tetramethoxymethane. (A) Solid at liquid nitrogen temperature. (B) Solutions in Ccl* (3600-1400) and CS, (1400-400). The cell lengths are as shown and the concentrations by volume are: (a) = 1:lO; (b) = 1:26; (c) = 1:60; (d) = 1:lOO; (e) = 1:600.
The structures of trimethoxymethane and tetremethoxymethane
303
configurations of the type shown in Fig. 3 are precluded as possible stable conformations. Thus with these two assumptions alone the possible number of isomers in trimethoxymethane was reduced to three, GGG, GGT and G’GT and in tetramethoxymethane to two, D, and A’, while in dimethoxymethane they would lead to three, TT, TG and GG and in the trivial case of methoxymethane to only one possible isomer. If now the lone-pair is considered as a discrete entity the possible interactions between the lone-pairs and attached methyl groups on adjacent oxygen atoms may be written as in Table 5 for the different configurations of the di-, tri- and tetramethoxymethanes and we see that the introduction of a third assumption, namely that the secondary effect affecting the stability of any conformation is that involving the lone-pair interactions, leads to the prediction of the stable conformations of both the dimethoxy (GG) and trimethoxymethane (GGT, G’GT). Thus we conclude that just as the lone-pairs appear to play an important role in determining the stable conformations of the normal esters [lo] they play no less a decisive role in determining the stable conformations of the orthoesters. authors wish to thank the Dairy Research Labor8tory of C.S.I.R.O. for the use of their Beckman IR-7 spectrometer and the Protein Chemistry Division of C.S.I.RO for the use of their Beckman IR-9 spectrometer. One of us (H. L.) also wishes to thank the Australian External Affairs Department for the 8w8rd of a Colombo Plan Fellowship.
AokmwZedgemente-The
[lo] N. L. OWENand N. SHEPPARD, Proc. C%m. Soo. (London) 264 (1963).
The structnres and vibrational spectra of trimethoxymethane and tetramethoxymethane H. LEE* and J. K. WILMSHURST Division of Chemkal Physics, C.S.I.R.O., Chemical Rese8mh Laboratories, Melbourne, h8trali8 (Received7 Apit
1960)
Al&r&--The infrared spectra of trimethoxymethane asd tetramethoxymethanein the vapour phase, 8s solutions in CS, end Ccl, and in the solid state have been obtained from 3-26 p together with the Reman spectra of their liquids. In solution or the vapour phase, trimethoxymeth8ne exists 8s a mixture of two isomers TBB, TBa’, the l8tter being stable by 610 cal/mole, while in the solid state the molecule assumes the An assignment has been m8de for ectchspecies. QCfCfconformation having point symmetry c, Tetrsmethoxymethsne hes S, symmetry in the solid and a distorted S, symmetry in solution and the observed spectra have been assigned on this b&sis. It is suggested that the principal effect in determining the stable conform&ions of the methoxymethanes, besides the steric hindrance of the methyl groups, involves the oxygen lonepair interactions.
ALTHOUQH the i&a-red spectrum of trimethoxymethane (trimethyl-orthoformate) has been reported [l], its Reman spectrum and the infrared and Raman spectra of tetramethoxymethane (tetramethyl orthocarbonate) have not been investigated previously. In both cases no structural determinations have been made although the thio analogue of tetramethoxymethane is believed to have a configuration having point symmetry S, in the solid [2]. In the present investigation the Raman spectra of the pure liquids and the infrared spectra of the solid, solution and vapour phases of the two esters have been obtained. The data for trimethoxymethane is consistent with the co-existence of two isomers in the liquid and vapour phases, both isomers having no symmetry, and the presence of a third isomer having point symmetry C, in the solid. By contrast, the data for tetramethoxymethane is consistent with the assumption of only a single isomer persisting in all three phases and having point symmetry 84 in the solid and a distorted S, symmetry in the liquid and vapour. The observed low energy conformations for both the dimethoxy- [3] and trimethoxymethanes can be predicted on the basis of interactions between lone-pair electrons on the oxygen atoms and represent an excellent example of the important contributions lone-pair interactions may make to the stable conformations of molecules. * Present address: Department of Chemistry, J&n
Sultan, Petabng Jaya, Malaysia.
[l] K. NWADA, J. Chena. Sot. Jupan 81, 1028 (1960). [2] W. G. PERDOK end P. TERPSTRA,Rec. Traw.C&n. I, [3] J. K. WILMSEURST, Cam.J. Chem. 86, 286 (1968).
347
687 (1943).
343
H. LEE and J. K. WILMSHURST
EXPERIMENTAL The trimethoxymethane was prepared from chloroform and sodium methoxide following the method outlined in Organic Synthesis [4] whilst the tetramethoxy derivative was synthesized by the procedure of TIECKELMANN and POST [5] using sodium methoxide and thiocarbonyl tetrachloride. The infra-red spectra of the vapours and solutions in carbon disulphide (1400-400 cm-l) and carbon tetrachloride (4000-1400 cm-l) were recorded using either a Beckman IR-7 or IR-9 spectrometer whilst the solid spectra were obtained with a Perkin-Elmer model 337 instrument. The solid and solution spectra are shown in Figs. 1 and 7 for trimethoxymethane and tetramethoxymethane respectively while the vapour spectra are presented in Figs. 4 and 6 respectively. The Raman spectra of the pure liquids were recorded photographically using a Hilger source and two prism glass spectrograph having 21 &mm dispersion at 4500 8. Mercury 4358 A radiation was used for excitation, the 4047 A radiation being filtered out by a saturated solution of sodium nitrate. Depolarization data were measured by the technique of EDSALL and WILSON [6] and corrected for convergence error by the method of RANK and KAIURISE [7]. The observed spectra and depolarization ratios observed for trimethoxy and tetramethoxymethane are tabulated in Tables 1 and 3 respectively, together with the observed infia-red data for the liquids and vapours.
Trimethoxymethane The possibility of rotational isomerism, giving rise to trans and gauche isomers (Fig. 2) about each of the three C-OMe bonds in turn, allows in theory twentyseven conformations for trimethoxymethane of which fortunately only seven,* the TTT, TTG, TGG, TGG’, TG’G, GGG and GGG’, are spectroscopically distinct. In addition steric interactions between the methyl groups in four of the conformations, TTT, TTG, CCC’ and TG’G, (Fig. 3) eliminate the need to consider them seriously as likely low energy isomers and we may accordingly confine our attention to the remaining three isomers only. The GGG conformation has point symmetry C, and hence the forty-five fundamental vibrations should divide into symmetry species as F,, all vibrations
= 15~ + 15e
being active in both the infra-red
and the Raman,
* The tlransor gaucheconfigurationabout any C-OMe hydrogen &tom is denoted T or B respectively.
with the type a
bond with respect to the methae
[4] H. GSMAN and A. H. Brxrr (Editors), “Organic Synthmb” Collective Vol. 1 (2nd Ed.), p. 258, John Wiley, New York (1961). [6] H. TIECKELMANN and H. W. POST,J. Org. Chem. 13, 265 (1948). [S] J. T. EDSALLand E. B. WILSON,J. Chern. Phys. 6, 124 (1938). [7] D. H. RAN-Kand R. KUARISE, J. Opt. Sot. Am. 40, 89 (1950).
349
The structures of trimethoxymethane and tetramethoxymethme Table 1. The infra-red and Raman spectra of trimethoxymethane Infra-red spectrum Solution (cm-l)
Int.
483
w
497
m
540
W-Ill
553
w-m
Vapour (cm-‘)
495 600 606
1
640 660 6671
Raman spectrum
Band type
A
C( TBB’)
cm-l
Relative inteneity -
912
8
929 -
8
466 983
1018 -
lOS0
-912 918 -928 1 936 -944 I
“8
8
8
1071
vo
1102
“8
1126
“8
1027 1033 1 1039 1045 10511 -1079 Ml084 ~1088 1092I 1110 1114 I
V1,) - v1, = VI1 - &,) = -%Ob
200 229
203 234 309
1 0 2
0.06
347
4
0.06
413 422 436 419
0 0 0 10
0.12
%. - Vl70 = VlS
494 -
6
0.19
yls
616 539
0.21
%b
563
0.21
%P
663
0.21 UO
-V280
VO66- %a = VP66- VWb=
NO
40
-ho
VI2 vg -vu vpar,-v,,. v230- h,b VP20-ho ;‘a~; - hb
683 696 742
11 6 1
766
3
0.64
871
0
C( T&Y)
911
13
0.08
VI1
C(TW)
927 -
16
0.08
f&
= = =
606 627 666 661
=
669
=
yII?L
VZ?. Vzsa --
x
v200 =
ha
-%Ob
12
0.16
loa
3
0
1070
6
0.14
1072
8
0.14
1106
7
0.16
%.b’ Yp6b
1126
6
0.29
%.a*Va.5. -
1016
619
%VO
2 AC
=
409 422 430
V28S
0.05 0.1 dp?
w -988 991 1002I
139 186
0
602 626 VW VW VW w
varo - %3a = vBIo - v*,~ =
186
694
I367 662 672 687
Assignment
Depol. ratio
VOS. -
=
878
966
H. LEE and J. K. WIUMZORST
360 Table
I (coti.) Infra-red speotrum
Solution (our1)
I&.
vapour (om-1)
Band type
1137 A
B
1160 1143 1 -1184
-1169
m
~1190
m
-1177 I -1171 ~11881 1194 1200 ~1204
1167
Ramen speatnun cm-1
1158
Relative intensity
10
Assignment
Depol. ratio
0.27
A
B
1190
7
0.08
-
1196 -1202 1226 lRs4
1852 ~1557 1388 -
-1434 1446
* m 8 m
8 m s
1208 I
1218 1229 ~1237 I
A?
C( TGW’)
1366 1363 -1369 ~1376 1
B
~1366 1366 ~1376
U( TtX.7)
1281
0.09
1261 1304 1322
NO
%Sb
1859
6
0.5
1367
6
0.6
1377
4
0.6
m *
-1444 -1447 1463 1 1460 -
VS veab+ vdsr= 1292 y,. + y, = 1327 -_
-VPld -VW
Vpsa+ v*aa= 1449 -B
%lbt
-hlb
A 1462
20
0.72
-
1466 ~1634 ~1641 1661 1676 1606 1703 1736 1767
B w w w w w w m w
1466 ~1466 I ~1466 1476 ~1486 I
C( TUB)
C(TBff’)
V6
1473 1641
0 0
0.69
%a*
yloIv2
Km.
+
VSBb = -
F
+
y1le0
1o
5 vl,, VI1 ;:
+vls
= =
+ %E. = + v; = fv, = $ y = IBa=
1534 1649
1664 1672 1623 1711 1736 1763
The structurea of trimethoxymetbane and tetmmethoxymethane Table
361
1 (cot&) Infia-red fql‘xtrum
Solution fcm-’
Int.
Raman spectrum
Baud
VCbpOUr lorr’l
tYlJe
am-l
Relative intetitv
Depol. ratio
1773 N1796 NM03
vmb
3000 ~3542 3560 ~3606 3648 3692 3802 3905 3937
1798 1812
VSlb + VIBb= vII +v,,~ = vI1 + vps,,= vI1 +vub =
vll
2108
+ va,,, =
+ vII.
=
,,,+G= 2197 v**. + v*e = 2199 Va3) + VIOL= 2220 vz3,,+ vIab = 2226 vzsL + vpw = 2298 vaJo+ vBso = 2322
~~~~=““S” 4&b= 2668 vIlb + v, = 2603 VW, + VBab= 2635
V,
+v,
V,
+
ye
=
2686 2734
%b
+
VPtb
=
2797
=
2890
-
2847 2854 ~2870 2879 ~2888
C( TOG’) 2842 2838I C( TOO)
2903 2913
2947
=
1822 1853 1892 1942 1983 2014
vg
2782 2834
2937
v%,~= 1789
ylo + v~,,, = -vIso + va,,, =
~2176 N2199 ~2208 2217 2299 2319 2336 ~2381 2422 ~2442 N2SSS 2600 ~2637 ~2647 2667 ~2686 2730
2883
+
vsD + v,,~
1811 1863 1886 1942 1979 2001 ~2033 2066 2109 ~2127 ~2137
28371
A&gnment
~2918 2929 2940 2956 2963 -2975
20
0.05
V@
3011 -
0
3011 3016 3018 ~3021 I
B
v4 -
2874
7
0.2
2
2909
7
0.2
2 XV6 2 xv,
= 2904 = 2934
2 x Vu
= 2946
O( TOW)
C( TOO’)
=
2949
16
0.1
3005
10
0.45
x vID
-
Vu VlW v1r -- Vl. 3640 3663 111 + s& = 3699
Vl
+v**b
Vl
+
=
vaea =
Va
+vnr.
=
3646
Vl
+ va,,
=
3696
II_
+
Vl
+v,=3912
vIIb =
3820 ^___
352
H.
Table
2. The
and J. K.
WILMSJXURST
approximate description and observed frequencies of the fundamental of the three conformations of trimethoxymethane
Species Cl
LEE
C*
Number
Approximatte description of vibration CH, extsrmal
CH, internal
Observed fundamental frequencies BBP
(2976) (2967) 2843 1473 1460 -1446 1237 1139 1106 926 697 601
V14 y15
0
vibrations
TU@V
TGO’b
(3016) (3011) (2963) (2847) (1473d) (1469) 1466 1261d (1143) 10706 936 5946 600
3016 3011 2963 2847 1476 1462d 14626 1229 1143 1086 918 660 4796
(3016) (3011)
3016 3011
(2963)
2963
(14736)
14736
(1462d) (1450) (1447) 1377d 1365 1226’ -1202’ (-1171) (1143) (1126) (1114) 1033 991 756 742 3476 309d
1462* 1450 1447 1367‘ 1366 1208 1197 ml171 1143 1126 1114 1086 1042 696d 687* 663d 5408
torsionc-0 (3001) 2978 ( 2974 2960 { 2964 1484 { 1479 1452 ( 1449 1380 ( 1374 1206 ( 1200 1169 ( 1163 1090 ( 1083 1008 1 1004 601 ( 594 648 <\ 643
a I&m-red frequencies for the solid. * Infra-red frequencies for the vepour unless indicated otherwise. 0 Fmquenoiea in parenthesis have else been assigned to fundamentala of the TM d Raman data for the liquid. * Liquid data.
isomer.
The structures of trimethoxymethane Table 3. The infrared and Raman
Int.
vapour (cm-‘)
457 465 470
462
SO0 577 684 690
581
of tetramethoxymethane
spectra
Raman spectrum
Jnfra-red spectrum Solution (cm-‘)
353
and tetramethoxymethme
655 662 675 694 729 755 793 99s 1017 1037 ~1072
(cm-‘)
Relative intensity
Assignment
Depol. ratio
111 140 165 199 301 318(P) 330 403
0
P ;Ipt
459
6
0.04
V41
47s 603 625
4
0.04
VI1
;
683
5
612
0
663 675 696
! 8
P? dp P
762 773
16 4
0.01 P
P
vao
P 053 dp
P 0.52
P VlO - v,~ = %s - VI1 = V7 - vtob= %a - v, = VlZ vam - %s =
VP7
V98 -
1110
t 6 2 6
1114
3
0.46
1161
3
0.54
1196
2
0.1
1265
0
P
1346 1378
0 1
P
1452
10
0.59
OT74
VbO =
169 200 302 320 401
529
VP0 V90a - vll = 608 2 x VI* = 660 V0 - vl* = 662 ?A6 %I - vp, = 693 %8 - VI1 = 7373
VlO VOS VI9
992 1016 1035 1076 1093
108
-vll=
+
v41=
792
VP %4
of)13
1124
1167
1214 1218 1224
1212 1258 133s
%l vl,, VI*
+vdo= -+ VI1 =
1336 1346
1442 ~1462 1466 1474 1655 1602 1610 1619 1676 1701 10
w w w w
a
0.41 l& +VI1 + vBa i_ v8, f vge + vg +
vi1 = v** = vile= v,~ = vao = vz, =
1668 1599 1618 1629 1691 1699
364
H. LEE and J. K.
WILMSHURST
Table 3 (cont.) Infra-red spectrum Solution (cm-~) -1769 1783 1864 -1876 NlOOS 1966 2049 2070 2087 -2109 2123 -2202 2236 2328 2386 2411 2426 2461 2660 2601 2665
1nt.
Raman spectrum v&pour (cm-~)
Relative intensity
(cm-‘)
Depol. ratio
w
V80)+ v*s = vga +vao= vsga+ v,, = v, + vg, = VI0 + vg, = % + vaa= v, + vpg= 2 x v,, = vzl + vssb = lag + v*go= “y ; r,, _=
1761 1793 1866 1878 1922 1060 2953 2074 2093 2110 ;2;7
va8 + v”,; : 2234 vs8 + vQ8= 2336 V, + ye4 = 2396 v, + vzJ = 2416 2 X vaB = 2424 v, + vQ8~ = 2464 v, + vs = 2614 vss + vQ, = 2609 vQ4 + v%,, = 2678 2844 2860 2866
2844
16
2877
3
P
2914
2919
6
0.06
2941
2961
20
0.02
2976
2982
4
0.76
2997
3006
8
0.44
2840 2884
-3070 3309 3412 3466 3666
Assignment
0.01
2
X vs6 =
21384
V,
+ va6 =
2894
X vsb =
2932
2
Vl, V16. %I
P vg,, + vlB =
w w
1~11 + val = vIo + vll = V~O + v&l =
w w
w
3327 3422 3472 3678
modes being polarized in the latter. Calculation* of the GERHARD and DENNISON [8] parameter gives /I = -0.40 and hence the infra-red vapour spectrum should consist of two types of bands, parallel bands having a medium-strong Q branch and a PR spacing of ~18 cm-l and perpendicular type bands having a strong Q branch and PR spacing of -13 cm-l. * The moments of inertia used in calculating the GEREARD and DENNISON or BADGER and ZVMWALT pwameters were calculated (assuming rCH = 1.09 A, Q-~ = 1.44 A and all angles tetrahedral) to be: HC(OCH,),
Btw TBff ( TBCi
C(OCH,).kz.
I*i 339.13 279.26 236.55
IV? 339.13 364.34 419.94
1st 501.67 523.51 510.18
514.36 463.09
626.61 453.09
626.51 744.32
t x 1040g-cms. [S] S. L. GEREARD and D. M. DENNISON,
Whys.
Rev. 43, 197 (1933).
The structures of trimethoxymethane and tetramethoxymethane
Fig. 1. The i&a-red spectrum of trimethoxymethane. Solutions in Ccl, (3900-1400) and CS, (1400-400). Unless otherwiseindicated the concentration is 1: 10 by volume and the cell length is as shown. (a): concentration = 1: 50 by volume. (B) Glass pbtained by rapid freezing from the vapor onto a plate cooled to liquid air temperature. (C) Solid obtained by crystallization of the glassy material at ~-55% with subsequent cooling to liquid air temperature.
(4
355
H. LEE and J. K.
356
WILMSHURST
6
tram
C”3
‘V,]”
,p
CH30y’
// . \
/ ‘-OCH3
/ ’
‘\
.
CHpu’
.LOCH3
: 1
gaucho Fig. 2. The
tramsand gauche conformations about SLC-O
bond in trimethoxymethane.
The TGG conformation has no symmetry and hence all forty-five fundamental vibrations should appear in both the infra-red and the Raman, all Raman bands being polarized. Calculation of the BADGER and ZUMWALT [9] parameters give S = 0.03 and p = 0.61 and hence the infra-red spectrum of the vapour should exhibit three types of band contours, type A having a medium Q branch and PR spacing of ~13 cm-l, type B having a PQQ’R type structure with a QQ’ separation of ~4.5 cm-l and type C having a very strong Q branch and a PR spacing of -19 cm-r. Although the TGG’ conformation should ideally have one plane of symmetry and therefore give rise to twenty-five polarized and twenty depolarized bands in the Raman effect, the presence of two pairs of directly opposed lone-pairs on the oxygen atoms involved in the GG’ configuration (Fig. 3) will almost certainly cause the two OCH, groups to twist away in opposite senses from each other thereby removing this symmetry plane and causing all vibrations to be polarized in the Raman effect. The BADGER and ZUMWALT [9] parameters for the TGG’ conformation are S = -0.63 and p = O-96 and hence the infra-red spectrum of the vapour should exhibit three types of band contours, type A having a P&R structure with a medium to strong Q branch and PR separation of ~13 cm-l, [9] R. M. BADGER
and L. R. ZUNWAET, J. Chem. Whys. 6, 711 (1938).
The struotures of trimethoxymethane
and tetramethoxymethane
B
A X
-4
Y
CH3
*/-
X=H Y=OC)5 I
dG
X=0 11 Y=H “‘3
.
Fig. 3. Conformations of trimethoxymethane (A) Illustrating the steric hindrance in the G’G and TT conformations. (B)JIllustrating the lone-pair interactions involved in the GG’ conformation. (C)Jirhe GGG conformation, point symmetry C,. The methane hydrogen atom protrudes upwards from the plane of the paper. (D) The GGT conformation.
Fig. 4. The infra-red spectrum of trimethoxymethane
vapour.
367
358
H. LEE and J. K.
WIIZSHURST
type B having a PR or possibly a PQQ’R structure with a separation of -9 cm-l (P R) or possibly ~5 cm-l (QQ’) and type C having a P&R structure with a medium Q branch and PR separation of ~18 cm-l. A comparison of the i&a-red spectrum of the solutions with that of the glassy solid obtained by rapid deposition from the vapour onto a plate cooled to liquid air temperature (Fig. 1) reveals a number of sets of bands displaying differences in relative intensities between the two phases and immediately suggests the presence of two rotational isomers. This is further substantiated by the behaviour with temperature of the relative intensities of pairs of bands, notably the pair at 912, 929 cm-l, in both the infra-red and Raman, characteristic of the co-existence of rotational isomers. With these critiques the bands in both the infra-red and Raman that can be definitely assigned to the stable and less stable isomers are given in Table 1 in bold characters and underlined respectively. The Raman spectrum of the liquid contains forty-five bands of which only one is depolarized, and that doubtfully, consistent with the presence of the unsymmetrical isomers TGG and TGG’. The infra-red vapour band contours of these isomers are virtually identical except for the type C bands which exhibit a strong Q branch in the case of the TGG isomer and only a medium strength Q branch in the case of the TGG’ isomer. A study of the vapour spectrum (Fig. 4) shows that when a band known to belong to the stable isomer in the liquid gives rise to a type C contour, the contour exhibits only a medium intensity Q branch whereas when a band known to belong to the less stable isomer in the liquid gives rise to a type C contour it exhibits a strong Q branch. Thus we may conclude that the stable isomer in the liquid has the TGG’ conformation. The variation in relative intensity with temperature of the pair of bands at 912, 929 cm-l then permits us to calculate that in the liquid state the TGG isomer is less stable* by 610 cal/mole. Crystallization of the glassy material gave rise to a solid having an i&a-red spectrum different from that of either isomer present in the glass or in the liquid, the most characteristic feature of which was the presence of a number of very sharp doublets (Fig. 1). This feature is typical of that expected of a molecule possessing a three-fold or higher rotation axis sitting on a crystal site of lower symmetry, the effect of which is to remove degeneracies causing each pair of vibrations that would normally be degenerate to appear as a very close doublet and we accordingly assume that the molecular species in the crystalline state has the CCC conformation. As a preliminary to the assignment of the observed spectral data it is convenient to classify the vibrational modes of trimethoxymethane into their approximate descriptions and in particular as to whether they are internal or external modes of the methyl groups as shown in Table 2. The CCC species. This species has point symmetry C, and, if it is assumed, as above, that the crystal site symmetry effectively removes the degeneracy of the type e vibrations causing each to appear as a closely spaced doublet, then we may immediately assign each such doublet to an e type mode. Thus we assign the doublets at 2976, 2957, 1482, 1451, 1377, 1202, 1166, 1087, 1006,597 and 546 cm-l * Infra-red studies give the value 622 c&l/mole whilst Raman studies give the value 603 cal/ mole.
The structures of trimethoxymethane and tetramethoxymethaaa
360
tov 17~ v 18, v19, v21? v221 v23, v24~ v25~ v28? v21 and V~ respectively and the remaining singlet bands at 3001, 2843, 1473, 1460, 1445, 1237, 1139, 1106, 926, 597 and 501cm-ltov,,v This assignment 4, v59 v6, v79 v89 vSt v107 v117 v12 and v13respectively. is given in Table 2. The TGG and TGG’ isomers. With the aid of models and the known inertial
l-
Y
H
2 0
A
Fig. 5. Projections of trimethoxymethane on the planes containing the zz and yz inertial axes. A = TQB isomer; B = TBB’ isomer.
axis of these conformers (Fig. 6) it is possible to roughly estimate the band contours for the different vibrational modes of each isomer and the assignment of the fundamentals given in Table 2 has been made on this basis. This assignment is necessarily somewhat arbitrary and accordingly it will not be discussed further. It does however lead to a satisfactory assignment of all the remaining observed bands as overtones or combination tones (Table 1).
H. LEE and J. K. WILMSHURST
360
Tetramethoxymethane If the methane hydrogen atom in trimethoxymethane is replaced by a methoxy group then only two of the seven configurations, the TGG' and TGG, will give rise to possible low energy conformations. Ideally these should have point symmetries D,, and S, respectively but in both cases the lone-pair-lone-pair interactions of neighbouring oxygen atoms reduces these to S4 and C, respectively. For the conformation with point symmetry S, (distorted D,,) the fifty-seven fundamental vibrations should divide into symmetry species as l?,, = 14a(R, p) + 15b(I;
R, dp) + 14e(I;
R, dp)
where R, I, p and dp refer to Raman active, infra-red active, polarized and depolarized respectively. Calculation of the GERH.ARDand DENNISON [8] parameter gives @ = -0.39 and hence the infra-red vapour spectrum should consist of two types of bands, parallel bands having a strong Q branch and PR spacing of ~15 cm-l and perpendicular type bands having a medium Q branch and PR separation of ~12 cm-r. For the conformation having point symmetry C, (distorted S,), all vibrations should be active in the infra-red and polarized in the Raman. The GERHARD and DENNISON parameter [8] is calculated to be /l = 0.22 and hence all vibrations should exhibit the same infra-red band contour having a medium-strong Q branch and a PR spacing of ~12-13 cm-l. The observed vapour spectrum (Fig. 6) supports the assumption of a distorted S, structure for the stable conformation of tetramethoxymethane in the vapour while the fact that the Raman spectrum of the liquid contains thirty-nine bands of which only three are depolarized, many of the polarized Raman bands having counterparts in the infra-red, supports the assumption of the same conformation in the liquid. The infra-red spectrum of the solid is similar to that of the liquid (Fig. 7) with the notable distinction that the band at 995 cm-l in the liquid is absent and we accordingly conclude that the molecule has strictly S, symmetry in the crystalline state. In addition nearly every band appears to consist of two components separated by ~1-4 cm-l indicative of at least two molecules per unit cell unrelated by a centre of symmetry. * The assignment of the observed spectra of vapour, liquid and solid tetramethoxymethane is given in Table 3, while the fundamentals are also presented in Table 4 together with their approximate descriptions in terms of internal or external vibrations of the methyl groups or skeletal vibrations. DISCUSSION The foregoing determination of the stable conformations of both tri- and tetramethoxymethane has been based on two important assumptions, firstly, that the rotational isomerism about a carbon-oxygen bond is directly analogous to that about a carbon-carbon bond and gives rise to trans and gauche configurations (Fig. 2) and, secondly, that the' primary interaction affecting the stability of any particular configuration is the steric elect involving the methyl groups, i.e. eclipsed * This feature would beconsistent with a crystal structure fortetramethoxymethane to that (P42,,) observed for tetrathiomethyhnethae [2].
identical
The structures of trimethoxymethane and tetramethoxymethane
361
Table 4. Approximate description and observed frequencies of the fundamental vibrations of tetramethoxymethane pecies
Approximate description of vibration
Number
S4
CH, external
r v1 *2
Solid
mE =ym
-
*a
*4 *4
*0
a < *7 *0
*o
VI1 *la
6000
*11 .
VO-OJI~
*o-o 8000
*lo
VI‘
toraionc-0
BOH* BOHS
*40
so00
v416000 v44 lv43 torsiono-0
bless otherwise indicated, infrared mum data for the liquid.
Fig.
6.
Observed frequency
CH, internal
toraionoH,_o vapour data is reported.
Liquid/Vapou+ 3006 2972 2968
1474c 1473 1462” 1378O l19ec 99d 766b 476O 33oc -
3009 2986 2966 1474 1448 1464 1260 1118 1033 1019 679 791 -
(3006) (2972) (2968) (1474)C (1462)b (1473) 126Eb 1141 Io37b 1o17b 676b 773c 600 -
3004 (2986) 2966 1482 1468 1441 1212 1168 1106 1096
(3006) (2972) (2968) (1474)C 1473 1460 1218 1169 1133 10930 { 10760 684 466 -
690 470 -
b Infra-red data for the liquid.
cm -I The ix&a-red spectrum of tetramethoxymethane vapour.
H. LEE and J. K. WILM~HURST
302
Table 6. Summary of the i&era&ions Conformation
Molecule
between substituents, including lone-pairs, on adjacent oxygen atoms Interactions between adjacent oxygens
1
TT TB GW
2(1-l)” + 2(1*1)” (1.1)” f (l.l)OL + (l.Me)” + (l-Me)a 2(1*Me)O + (I*l)cI + (MeMe)=
L
BGW GQT BB’T
3(1-l)” -I- 3(1.1)” + 3(1*Me)” + 3(1.Me)LL 2(1+1)” + 3(1*1)” + 4(1*Me)” f 2(1-Me)a + (Me-Me)O: 2(1-l)’ -I- 4(1.1)” + 4(1*Me)” + 2(Me-Me)a
CH,WH,),
CWOCH,),
4(1*1)” -I- 8(l*l)c( + S(l-Me)’ + 4(Me*Me)= % 4(1*1)” + 6(l.l)a + 8(1*Me)” + 3(1-Me)O: + 3(Me*Me)OL ( s4 The supersoripts distinguish substituents directly opposed (“), i.e. lying in the same plane &B the oxygen atoms, or staggered (“), i.e. the planes oontGning the substituents and the oxygen atoms subtend an angle of -120” where they intersect on the join between the pair of oxygen WC&&
atoms.
cm-’
Fig. 7. The ix&a-red spectrum of tetramethoxymethane. (A) Solid at liquid nitrogen temperature. (B) Solutions in Ccl* (3600-1400) and CS, (1400-400). The cell lengths are as shown and the concentrations by volume are: (a) = 1:lO; (b) = 1:26; (c) = 1:60; (d) = 1:lOO; (e) = 1:600.
The structures of trimethoxymethane and tetremethoxymethane
303
configurations of the type shown in Fig. 3 are precluded as possible stable conformations. Thus with these two assumptions alone the possible number of isomers in trimethoxymethane was reduced to three, GGG, GGT and G’GT and in tetramethoxymethane to two, D, and A’, while in dimethoxymethane they would lead to three, TT, TG and GG and in the trivial case of methoxymethane to only one possible isomer. If now the lone-pair is considered as a discrete entity the possible interactions between the lone-pairs and attached methyl groups on adjacent oxygen atoms may be written as in Table 5 for the different configurations of the di-, tri- and tetramethoxymethanes and we see that the introduction of a third assumption, namely that the secondary effect affecting the stability of any conformation is that involving the lone-pair interactions, leads to the prediction of the stable conformations of both the dimethoxy (GG) and trimethoxymethane (GGT, G’GT). Thus we conclude that just as the lone-pairs appear to play an important role in determining the stable conformations of the normal esters [lo] they play no less a decisive role in determining the stable conformations of the orthoesters. authors wish to thank the Dairy Research Labor8tory of C.S.I.R.O. for the use of their Beckman IR-7 spectrometer and the Protein Chemistry Division of C.S.I.RO for the use of their Beckman IR-9 spectrometer. One of us (H. L.) also wishes to thank the Australian External Affairs Department for the 8w8rd of a Colombo Plan Fellowship.
AokmwZedgemente-The
[lo] N. L. OWENand N. SHEPPARD, Proc. C%m. Soo. (London) 264 (1963).