Subject: More re MOPAC IR

From: [email-masked] (Hr Dr. S. Shapiro)
Date: Fri, 5 Jul 1996 09:07:18 +0200 (MEST)

Dear Colleagues;

     A few days ago I posted a query about how to implement MOPAC for the gene-
ration of IR spectra.  I thank the handful of people who took the time and ef-
fort to respond.  In a nutshell, one simply has to add the keyword FORCE, and
a vibrational analysis will be added to the MOPAC output.  (n.b.: In my imple-
mentation of MOPAC 6.0, at least, the keywords FORCE and EF seem to be incompat-
ible.  Does anyone know why that might be?)   One responder mentioned a very
nice programme by Dr. J. Gosper of Brunel University (U. K.) which visualises
the output structure from a MOPAC .out file and which can animate one vibration-
al frequency at a time.  Unfortunately, I do not believe that there yet exists
a programme that will regenerate an IR spectrum as such from the output of a
MOPAC "FORCE" calculation.  (If I am wrong, kidly correct me.)

     As a result of the responses received, I was able to undertake an intelli-
gent search through the local science libraries, and found a particularly use-
ful article:

     SEEGER, D. M., KORZENIEWSKI, C., and KOWALCHYK, W., Evaluation of
     vibrational force fields derived by using semiempirical and ab
     initio methods.  J. Phys. Chem. 95: 6871-6879.

which lists experimental, semiempirical, and ab initio force constants and fre-
quencies for a variety of molecules, as well as recommended sacling factors for
different types of vibrations calculated using the MNDO, AM1, and PM3 parametri-
sations.  One of the responders mentioned that a Dr. Radom (Leo Radom?) has pub-
lished, or has in press, an article further detailing vibrational frequency
scaling factors for MOPAC- and ab initio-calculated molecular vibrational fre-
quencies.  However, I have been unavble to locate such an article.  Does anyone
out there know about this paper?  If so, kindly post details.

     I now wish to follow up my original query about MOPAC and IR with these
two questions:

     (i) The article cited above by Seeger et al. calculated MNDO, AM1, and PM3
scaling factors for hydroxyl O -- H stretches based only upon water.  I am par-
ticularly interested in a scaling factor for hydroxyl O -- H stretches in phenol
and phenolic-type molecules,  both  simple (cresols, xylenols, methoxyphenols)
and more complex (terpenoids, oestrogens, &c.).  Has anyone seen an article in
which a scaling factor for phenolic O -- H stretches is provided?

    (ii) In connection with the above, I am seeking values for phenolic O -- H
stretching frequencies with reference to particular orientations of the phenolic
hydroxy moieties vis-a'-vis the rest of the molecule.  For example, in an old
paper by Simonyi et al [Tetrah. Lett. (19): 1631-1632 (1975), the authors sug-
gest that for 2,6-di-isopropylphenol the O -- H peak at 3641 cm-1 corresponds
to the structure in which the hydroxy moiety is captured between the two methyls
of the isopropyl unit, whereas the O -- H stretch at 3619 cm-1 is due to "a"
conformation in which the hydroxy moiety is _not_ captured between a pair of
isopropyl methyl units.  Can anyone provide me with additional, especially more
recent, references providing experimental phenolic O -- H stretch frequencies as
a function of phenolic conformation?

     Thanks in advance to all responders.

With kindest regards,

(Dr.) S. Shapiro
Inst. f. orale Mikrobiol. u. allg. Immunol.
Zent. f. Zahn-, Mund- u. Kieferheilkd. der Univ. ZH
Plattenstr. 11
Postfach
CH-8028 ZH 7

Internet: [email-masked]
FAX-nr: ( ... + 1) 261'56'83

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From: A J Turner <[email-masked]>
Date: Fri, 5 Jul 1996 12:17:32 +0100 (BST)

Hi!

FORCE and EF are incompatable as FORCE auto uses BFGS to optimise if the
gradients are too large.  If you wish to use EF, optimise first, then take
the output and FORCE that.

Cheers

Alex

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|Alexander J Turner         [email-masked]                  |
|Post Graduate              |http://www.bath.ac.uk/~chpajt/home.html|
|School of Chemistry        |+144 1225 8262826 ext 5137             |
|University of Bath         |                                       |
|Bath, Avon, U.K.           |Field: QM/MM modeling                  |
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