Subject: CCL: AMPAC - FORCE Calcs

From: Mr Andrew D Allen <[email-masked]>
Date: Wed, 22 Jun 94 12:14:06 BST

I am currently doing FORCE calcuaions to reproduce IR / Raman experimental data

I would like to clarify a few points on the interpretation of the output.

	eg: of output from AMPAC (v2.1)

   I        67        68        69        70        71        72
 FREQ(I) 1366.7319 1390.1030 1395.4407 1423.7041 1441.6861 1449.8304

 MASS(I)   3.77921   3.06115   1.11094   3.79245   2.26716  10.52206

 DIPX(I)  -0.32584  -0.46142  -0.27128  -0.49568  -0.11089   2.31958
 DIPY(I)   0.31422   0.50441   0.55722   0.53976   0.47027   3.47399
 DIPZ(I)  -0.10781   0.07567  -0.15388  -0.24481   0.17432   3.01794

 DIPT(I)   0.46532   0.68780   0.63857   0.77264   0.51365   5.15335

............cut out

 VIB. 67    ATOMS  C  9  AND  H 10  SHIFT  0.75  ANGSTROMS    4.1%  RADIALLY
 FREQ.    1366.73  C 23       H 29         0.38               2.6%          

 VIB. 68    ATOMS  C  4  AND  H  5  SHIFT  0.41  ANGSTROMS    3.7%  RADIALLY
 FREQ.    1390.10  C 13       H 14         0.55               0.2%          
                   C 13       O 15         0.34              94.7%          
                   C 13       H 17         0.66               0.2%          

 VIB. 69    ATOMS  C  1  AND  H  2  SHIFT  0.44  ANGSTROMS    0.0%  RADIALLY
 FREQ.    1395.44  C  4       H  5         0.46               2.2%          
                   C 13       H 14         0.66               1.5%          
                   C 13       H 17         0.58               4.3%          

	1) Looking a VIB. 68, I read this as a C-O bond stretch as the radial
component is high for C13 to O 15. The stretch causes the atoms to move 0.34
Angstroms apart from each other.

	2) Some of the output details SHIFTs that are equivalent to bond
lengths, in the output above under VIB. 67 the C9 to H10 bond SHIFTS by 0.75
Angstoms. Is this the distance that the Hydrogen moves away from the carbon ???
If it is then is this not a bit large. If it isn't then what does SHIFT refer
to?
 	3) I can understand when I have a bond stretch, as the radial component
is high in proportion to the tangential motion, eg with VIB 68 we have 94.7%
radial motion. But how do I determine the difference between a "Wag" and a
"Scissor" which varies tangentially, in different directions?

	A few general points which seem fairly obvious:

Since these calculations are all in the gas phase, h-bonding and charge
interactions between molecules and solvent don't appear in the calculations.
Therefore the broadening of -O-H stretches etc won't be seen.

Also for complex (non-symmetrical) molecules there will be a large number of
vibrations which will involve a large number of bonds, how does one go about
assigning these vibrations to experimental data?

I look forward to hearing your replies and hope for some good advice, for
the interpretation of FORCE ouput.

Andy

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