A cautionary note on energy minima derived from force field calculations

Tetrahedron Letters No. 9, pp 819 - 820, 1978.

A CAUTIONARY J.M.A. Laboratory (Received The conditions

NOTE ON ENERGY MINIMA

Chemistry,

be made as to enerqy minima

Recently,

comparisons

minimization

osawa,

Collins,

obtained

University,

minimization

methods.

questioned

Several

Allinger's

the gauche

conformers

Conformational

hydroqen

hypothesis3

for some conformations

presented obtained

using the

is a real minimum;

if

by us using the Engler

since we can reproduce

both geom-

of 2,3-dimethylbutanea. b

(A)

transition

gauche

c2 70.72'

'2h 180.00°

(G)

(53.4')

G+A

statesb

G-G

c2 119.55O

c2v o.oo"

stretch

0.574

(0.55)

0.567

0.908

bend

1.190 (1.23)

1.501 (1.65)

1.774

2.975

twist

0.310

(0.27)

0.317

(0.16)

1.885

1.704

non-bonded

4.238

(4.23)

3.869

(3.93)

4.498

6.408

total

6.312

(6.32)

6.226

(6.29)

8.724

11.995

0.497

(0.44)

0.000

(0.00)

2.498

6.180

energyd

be-

are based upon the comparison

of 2,3-dimethylbutane

conformation

this force field correctly

anti

Steric energy

gauche

of their arguments

minima

!d H-C-C-HC

by us using the

I.

that we applied

Point group

cal

note must

This can be demonstrated

energy minima with those obtained

of the anti and gauche

We doubt whether

I. Data calculated

1978)

method.

force field are given in Table

Table

9 Jauar~

in force field calculations

is used'. A cautionary

approach

so, it is not the only and lowest one. Data on this compound

We are convinced

Delft, The Netherlands

energy maxima

by other minimization

and Schleyer'

components

Engler force field4.

Technical

and oartial

of published

cause of its force field dependence. of steric enerqy

FROM FORCE FIELD CALCULATIONS

In UK 15 December 1977; accepted for ptblicatlon

only be checked when the Newton-Raphson

Newton-Raphson

DERIVED

B. van de Graaf, A. van Veen, and B.M. Wepster

Baas*,

of Organic

for true energy minima

by the following

Pergamon Press. Printed m Great Britain.

(0.58)

0.540

a Energy data in kcal mole -I, conformational energy at 25'; data in parentheses from ref. 2. b The final second derivative matrix gives six eigenvalues equal to zero and in the case of transition states also one value less than zero. ' Dihedral angle about the central bond. d The conformational energies of the gauche conformer and the gauche + anti transition state have been corrected

by RT In 2. The data are qiven relative to the gauche conformer. 819

No.

820

etry and sterlc enerqy in ref. 4 exactly. 0.96 kcal mole-' The results gauche

in Table

conformations

reported3

dihedral

The second example

the central transition

1973 force field

minimum.

concerns

the energy

state between

the two axial

The "minimum"

bond did

I.

of axial tert-butylcyclohexane.

In the

1 and 4 of the cyclohexane

ring and

of ref. 2 is what we find to be the

tert-butylcyclohexane

for some conformations

enantiomers.

Relevant

data are

of tert-butylcyclohexanea.

Point group 0 H-C-C-CC

b

transition axial

(E)

equator&Z

c1 171.74O

c1 158.96'

12.661

17.390

steric energy

Strain energy

a-b

to the one

about the central

states are given in Table

the carbon atoms

minima

Conformational

the values

II.

II. Data calculated

Total

between

bond. Our value is almost equal

(about 72'). Rotation

and geometry

group6.

but those for the

identical5

is the difference

Data on the transition

plane through

atom of the tert-butyl

are almost

Most striking

angle about the central

is no mirror

given in Table Table

significantly.

using the Allinqer

there

from the Cartesian coordinates giver -1 lower with differences up to

0.056 kcal mole

I for the anti conformer

differ

not reveal any additional

minimum

of trl-tert-butylmethane

we find the minimum

in the sterlc energy components.

given

of the H-C-C-H

components

However,

9

energyd

5.421

(5.45)

10.150

0.000

(0.00)

4.729

(A)

stateb

A-+A

C &ooo 18.087 (10.87)

10.857

(5.42)

1.118

See note a-b of Table

and the tert-butyl formers

I. ' Dihedral angle about the bond between the cyclohexane ring d group. The conformational energies of the equator&Z and arc&Z cun-

have been corrected

equator&z2

by RT In 2. The data of the axial conformer

one, those of the axial + ax&z2 transition

1. D.H. Faber and C. Altona,

state relative

are relative

to the axial

to the

conformer.

Cornput. Chem. 2, 203 (1977); we thank Dr. Altona for a copy of

the manuscript. 2. E. osawa, J.B. Collins,

and P. v. R. Schleyer,

3. D.H. Wertz and N.L. Allinger, 4. E.M. Engler,

J.D. Andose,

5. The Cartesian

coordinates

6.348

Tetrahedron

ii,

Tetrahedron

and P. v. R. Schleyer, of the

kcal mole -' and a H-C-C-H

anti

6. B. van de Graaf and B.M. Wepster,

J. Am.

conformation

dihedral

22,

Chem.

Lett.

Sm.

%,

8005

(1973).

from ref. 4 give a steric energy of

angle of 175.83';

Tetrahedron

2667 (1977).

1579 (1974).

24,

it is not a minimum.

2943 (1975).