Concise Physical Chemistry

c20 JWBS043-Rogers September 13, 2010 11:29 Printer Name: Yet to Come

HETEROATOMS: OXYGEN 333

# STO-3G opt=z-matrix

methanol

01

C

O,1,R1

H,1,R2,2,A

H,1,R2,2,A,3,D1

H,1,R2,2,A,3,D2

H,2,R2,1,A,3,D3

Variables:

R1=1.5

R2=1.1

A=110.

D1=-120.

D2=120.

D3=60.

FILE 20.9 Input file for a GAUSSIAN©CSTO-3G calculation on methanol. The dihedral

angle D3 accounts for the refusal of O to conform to the H C C plane.

opt=z-matrixrequests optimization in internal coordinates and output in

z-matrix format. Construction of the z-matrix input geometry goes as follows:

Take carbon as the reference atom. Call it atom 1. The oxygen atom is attached to

the carbon atom at a distance R1. Three hydrogen atoms are connected to C1, each at

a distance R2. Distance R1 is specified as 1.5Ainthe ̊ Variables:section of the

input file and distance R2 is assigned the value 1.1A. Bond distances are guesses ̊

taken from prior experiments, possibly spectroscopic studies.^3 The simple O C H

angles are guessed to be pretty close to the tetrahedral angle of methane, A∼= 110 ◦,

also entered into theVariables: section of the input file. Along with the simple

angles, there are two dihedral angles D1=–120◦and D2= 120 ◦relative to atom 2.

Finally, there is a hydrogen (atom 5) attached to O (atom 2) at a bond length of 1.1A ̊

and a simple angle A∼= 110 ◦. This is the OH hydrogen that is out of the H C O

plane at a dihedral angle of D3= 60 ◦.

C - O

There is no external coordinate system for thez-matrix input file. The position of

each atom is given only relative to some other atom or atoms in the molecule. The

input geometry is said to be given ininternal coordinates.

(^3) Typical spectroscopic bond lengths are 1 to 2A (Chapter 18). Electron diffraction studies yield 1.1 ̊ Afor ̊
the C H bond length in 1,2-dichloroethane in the vapor phase.