Determined by Electrostatic Energy Calculations

1358 N o tizen

Hydrogen Bonding in M

0 2C12 • H 20

Determined by Electrostatic Energy Calculations

We r n e r

H.

D ep artm en t o f G eological Sciences, U n iv ersity o f Illinois, Chicago, Illinois

(Z. Naturforsch. 32b, 1358-1359 [1977]; received July 19, 1977)

E lectro sta tic E nergy C alculations, B ifu rcated H ydrogen B onds

T he configuration o f least electrostatic energy for th e hydrogen atom s in both p o ly ­ ty p e s o f M⁰O²CI² • H 20 w as obtained b y sy stem a tic variation o f th e orientations o f th e w ater m olecules. T he internal geom etry o f th e H 20 group w as k ep t con stan t throughout th e variation. The hydrogen bonds are o f th e

/'Cl.

bifurcated ty p e: 0(w )-H (w )C v "

Nvl

Recently

proposed to discriminate between different hydrogen bonding arrangements in the two polytypes of

•

2-3 by cal­

culating the electrostatic lattice energy for three different hydrogen atom positions in the P m n2i structure 2 and for two alternative arrangements in the Pm nb stru ctu re3. Such approach can lead to false conclusions if none of the alternatives corres­

ponds to the extreme value of the electrostatic lattice energy.

I t is possible to find the position of least electro­

static energy by varying systematically the orien­

tation of the water molecules while keeping the dimensions of the molecules themselves constant4.

I t was found th a t 28 hydrogen atom positions in 6 hydrates calculated in this way agreed on the average within 0.09 Ä with positions determined by neutron diffraction4. In several cases predicted hydrogen atom positions have been subsequently

R eq u ests for reprints should be addressed to Prof.

W . H . Ba u r, D epartm en t o f G eological Sciences, B o x 4348, U n iv ersity o f Illinois, Chicago, Illin o is 60680, U SA .

verified by neutron diffraction and by nuclear mag­

netic resonance5.

In both polytypes the oxygen atoms of the water molecule, 0(1), are located on mirror planes. The positions of the hydrogen atoms were varied by rotation of the plane of the water molecule normal to this mirror plane. The dimensions of the water molecule were assumed to be 0(w)-H(w) 0.96 Ä and <H (w )-0(w )-H (w ) 109° corresponding to the mean of these values averaged from 81 water molecules determined by neutron diffraction5-6. The extreme value of the lattice energy was found in every case when the plane H(w)-0(w)-H(w) was almost parallel to the crystallographic plane (010) (Table I). These positions are about halfway be­

tween the positions named H(2) and H(3) by

d e r1.

This corresponds to a bifurcated arrangement in which each hydrogen atom is approximately equidistant from two Cl atoms at about 2.7 Ä and the angles 0(w)-H(w)-Cl are very bent (129 to 138°). The atoms Mo, O(w) and both hydrogen atoms of the water molecule are in a planar arrange­

ment.

T able I . C oordinates o f hydrogen positions o f least electrostatic energy in tw o p o ly ty p es o f M0O2CI2 • H2O.

C om pound A tom X y ^z

P m n2i M0O2CI2 • H2O P m n b M0O2CI2 • H 20 P m n b M⁰O²CI² • H²O

H (w ) H (w 1) H (w 2 )

0.5847 0.0847 0.0847

0.7565 0.5669 0.3129

0.3386 0.6131 0.8924

Similar geometries have been found previously.

In MnCl

• 4 H 2 0 7-8 atom H(12) forms a bifurcated bond of similar dimensions. Also in MnCl

* 4 H

O two of the w ater molecules [0(3) and 0(4)] have planar arrangements of Mn, 0(w) and the hydrogen atoms. This example is also instructive because the hydrogen atom positions were predicted on the basis of electrostatic calculations in 19657 and confirmed by neutron diffraction by

and

in 19718. I t is expected th a t the hydrogen atom positions predicted here for the two polytypes of M

O

CI

• H

O are within 0.1 Ä of their true posi­

tions as they can be determined by neutron diffrac­

tion.

I th an k th e C om puter C enter o f th e U n iversity o f Illin o is for com puter tim e.

Table H . E nvironm ents o f hydrogen atom s in tw o p o ly ty p e s o f M0O2CI2 ■ H2O.

a) P m n 2i

b) P m nb

H (w )-C l 2 .602Ä 0 (1 )-H (w )-C l 137.5°

H (w )-C r 2.738 0 ( i ) - H ( w ) - c r 128.9

H (w )-M o 2.964 M o -O (l)-H (w ) 125.5

H (w l)-C lI I 2 .6 7 0 Ä H (w 2 )-C ll 2.667 A

H ( w l) - C ll 2.725 H (w 2 )-C lII 2.706

H (w l)-M o I 2.918 H (w 2 )-M o II 2.893

0 ( l ) I - H ( w l) - C l I I 137.8° 0 ( 1 )H -H (w 2 )-C lI 136.8°

0 (1 )I -H ( w l )-C lI 130.0 0 (1 )II-H (w 2 )-C 1 H 130.1

M o I - 0 (1 )I -H (w 1) 125.4 M o H - 0 (1 ) I I - H (w2) 125.4

N otizen 1359

1 F . A . S c h r ö d e r , Z. N aturforsch. 3 2 b , 361 [1977];

in th is paper th e positional param eters o f H 2 II sh ou ld read 0.1, 0.65, 0.6 and n ot 0.1, 0.65, 0.82 (private com m unication from F. A. S c h r ö d e r ) . 2 F . A . S c h r ö d e r and A. N0R l u n d C h r i s t e n s e n , Z.

A norg. A llg. Chem. 392, 107 [1972].

3 H . S c h u l z and F . A. S c h r ö d e r , A cta Crystallogr.

A 29, 322 [1973].

4 W . H . B a u r , A cta Crystallogr. 19, 909 [1965]; pro­

gram for calculating electrostatic energies, MANIOC.

5 W . H . B a u r , A cta C rystallogr. B 2 8 , 1456 [1972].

6 T h e 0 (w )-H (w ) d istan ces assum ed b y Schröder range from 1.3 to 1.9 A , th e angles (H w (-O (w )-H (w ) v a ry from 57 to 93°. N on e o f th ese valu es are real­

istic.

7 W . H . B a u r , Inorg. Chem. 4, 1840 [1965].

8 Z. M. E l S a f f a r and G. M. B r o w n , A cta C rystallo­

gr. B 2 7 , 6 6 [1971].

Effect of Ortho Substitution on the Aminolysis of Active Esters in Aprotic Solvents

T.

A. F.

F.

Central R esearch In stitu te for C hem istry o f th e H ungarian A cadem y o f Sciences,

H -1 525 B ud ap est, H ungary

M. L

w, and L.

Chemical W orks o f Gedeon R ich ter L td ., H -1 103 B u d ap est, H ungary

(Z. Naturforsch. 32b, 1359-1360 [1977]; received August 8, 1977)

K in etics, A m inolysis, A ctive E ster, A protic S olven t, Ortho E ffect K in etics o f th e piperidinolysis o f active esters o f acetic acid in acetonitrile and chloro- benzene w as in vestigated . The rate data show intram olecular catalysis for th e am inolysis o f

2-nitro- and 2-cyanophenyl esters, w hile reactions o f 2,6-disubstituted com pounds are hindered b y steric inhibition.

The active ester procedure of amide bond forma­

tion plays an im portant role in peptide syntheses.

From the time of the introduction of this technique a number of leaving phenoxy groups have been examined many of them containing ortho substi­

tu e n t^ ) 1. Though the activation observed when introducing electron-withdrawing substituents into the m eta and para position of the leaving group was supported by kinetic studies2, the effect of ortho substitution on the kinetics of amide bond formation has not been investigated so far.

This paper presents kinetic data on the aminolysis

R eq u ests for reprints should be sen t to D r . F.

D u t k a , Central R esearch In stitu te for Chem istry o f th e H ungarian A cadem y o f Sciences, H-1525 Budapest, H ungary.

of several active esters in two aprotic solvents. For this study active esters of acetic acid were used3 because a) aminolysis of carboxylic acid esters (including aminoacyl derivatives) proceeds through a common mechanism4, and b) intramolecular interactions between the leaving group and the acyl portion (as it m ay occur when using esters of amino- acids5) are absent.

Under the pseudo first order conditions of excess amine all the reactions followed the general rate equation for ester aminolyses in aprotic solvents6:

d [ester]

--- ^ ---- = (k

[amine] + k

[amine]2) [ester].

Our second and th ird order rate constants together w ith literature p K a values for the leaving phenoxy groups are summarized in the Table.

In accord with the early observation th a t aminolytic reactivity of active esters strongly depends on the basicity of their leaving group8, with th e exeption of 2,6-disubstituted compounds which exhibit negative deviations, logarithms of k

AN and k

CB values in the Table can be correlated linearly w ith the

of the leaving groups (not shown).

Since the extent of the lag behind the expected rates for the aminolyses of 2,6-disubstituted re­

actants is related to the steric requirements for the o,o'-substituents (H < F <C1 < C H

« B r), steric in­

hibition is th a t hinders the aminolysis in these cases.

The facts th a t k

AN and k

CB for the reaction of 2-nitro- and 2-cyanophenyl acetate fit the above correlation, while k

CB constants are considerably higher th an expected (leading to slight solvent dependences for k

and low k

CB/k

CB ratios), clearly indicate7 anchimeric assistance by the o-nitro- and o-cyano functions. These results provide the first example of intramolecular participation by an o-cyano group in ester aminolyses, and suggest th a t aminolytic reactivities of 2-nitrophenyl esters of aminoacids exceeding those of the 4-nitro analogues are, a t least in part, due to the in tra­

molecular catalysis by the o-nitro group.

1360 N o tizen

Table. Second and third order rate co n sta n ts for th e reaction s o f esters w ith piperidine in acetonitrile (k2AN and ks^N) and chlorobenzene (k2CB and k3CB) a t 25 °C; p K a v a lu es for th e leavin g p h en oxy groups in w ater at 25 °C.

E ster p K a ^{k 2AN}

M -is-1

k 3AN

M -2s^{- 1}

k 2CB

M -iS- i

k 3CB

M -2s - i

PhO A c I0.0 0a 3.9 10- 5 5.6 ^{1 0 - 5}

_ _

4-Cl-PhOAc 9.42a 8.5 10- 4 1.5 ^{1 0 - 3} 1.0 io-4 8.3 I0- 4

4-CH3OCOPhOAc 8.47b 2.70 IO" 2 3.2 1 0 - 2 1 . 2 1 0 - 3 1.6 IO- 2

4-CH3COPhOAc 8.05b 4.37 IO- 2 c 1.05 10- 3 2.40 10- 2

4 -N 02P h 0 A c d 7.15e 2.19 ^c 3.50 10- 2 8 . 2 10-i

2 -F -P h O A c 8.70* 1.51 10- 2 2.52 10-2 1.0 10-3 1.26 1 0 - 2

2-C l-PhO A c 8.53a 1.15 10- 2 2.07 io- 2 7.5 10- 4 1.42 10- 2

2 -N 02- P h 0 A c 7.23e 2.26 ^c 5.30 10-1 1.53

2-C N -PhO A c 7.18f 2.67 ^c 6.08 1 0 - 1 2.07

2,6-Cl2-P h O A c 6.79f 2 . 8 8 1 0 - 1 c 2.58 10- 2 7.6 IO- 3

2,6-(C H³)²-4 -N 0²- P h 0 A c d 7.078 2.44 10- 3 c _{1 . 2 1 0 - 4} 1.5 10- 4

2,6-Cl2-4 -N 02- P h 0 A c 3.55e 64.2 ^c 1 1 . 0 c

2,⁶-Br²-⁴-N^{0 2}- P h⁰Ac 3.38e 13.4 ^c 4.49 ^c

F s P h O A c d 5.53h 73.0 ^c 4.70 ^c

Cl5PhO A cd 4.821 15.7 ^c 2.29 ^c

a A . I. B i g g s a n d R . A . R o b i n s o n , J . C h e m . S o c . 1961, 390; b P. M . G . B a v i n a n d W . J . C a n a d y , C a n . J . C h e m . 35, 1555 [1957]; c u n d e t e c t a b l e t h i r d o r d e r r a t e c o n s t a n t s ; d r a t e d a t a t a k e n f r o m r e f . 7; e J . J u i l l a r d , C . R . A c a d . S e i . S e r . C . 262, 241 [1966]; f J . - C . H a l l e , R . H a r i v e l , a n d R . G a b o r i a u d , C a n . J . C h e m . 52, 1774 [1974]; s A . F i s c h e r , G . J . L e a r y , R . D . T o p s o m , a n d J . V a u g h a n , J . C h e m . S o c . B , 1966, 782; h J . M . B i r c h a l l a n d R . N . H a s z e l d i n e , J . C h e m . S o c . 1959, 3653; 1R . A . R o b i n s o n a n d R . G . B a t e s , J . R e s . N a t . B u r . S t a n d .

A 70, 553 [1966].

1 M . B o d a n s z k y , Y . S. K l a u s n e r , and M . A.

O n d e t t i , P ep tid e Syn th esis, W iley, N ew Y ork 1976.

2 H . R . K i r c h e l d o r f , E . S t e n g e l e , and W . R e g e l ,

L iebigs A nn. Chem. 1975, 1379.

3 K in etic arrangem ents follow ed p reviou s lines, T.

K ö m i v e s , A. F. M a r t o n , and F . D u t k a , Z. N atu r - forsch. 31b , 1714 [1976].

4 H . J . J o n e s , Chem. In d . (London) 1974, 723.

5 M . B o d a n s z k y , M . L. F i n k , K . W . F u n k , M .

K o n d o , C. Y . L i n , and A. B o d a n s z k y , J . Am.

Chem. Soc. 96, 2234 [1974].

6 D . P . N . S a t c h e l l and 1 .1 . S e c e m s k i , J . Chem. Soc.

B 1969, 130.

7 L . K i s f a l u d y , M . L ow , G y . A r g a y , M . C z u g l e r ,

T . K ö m i v e s , P . S o h ä r , and F . D a r v a s , in “P ep tid es 1976” , p. 55, Proc. X I V t h P ep tid e Sym posium , W epion, B elgium 1976.

8 J . P l e s s and R . A . B o i s s o n a s , H elv . Chim. A cta 46, 1609 [1963].

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