Subject: isocyanates: summary

From: [email-masked] (Douglas A. Smith Ph.D.)
Date: Thu, 31 Oct 1996 13:14:45 -0500

Quite a while back I asked a question of the list regarding the calculated
non-linearity of isocyanates, i.e. R-N=C=O.  I did get a reasonable
response, and apologize for the summary not appearing earlier (we changed
Internet access providers and were without mail access for over a month!).
Anyway, here is my original posting and the edited responses.

The answer, in short, is that the bond is NOT linear.  I hope that those who
answered that it should be linear, or thought that it should be linear, are
not offended.

Doug

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My original posting:

However, the results are not intuitively obvious.  For example, using
H-N=C=O as a test case, the bond angles and distances are as given in the table.
 
                       distance                  angle
                 N-C             C-O             N-C-O
               -------         -------         ---------
 
         AM1    1.23 A          1.20 A           166.7
 
         PM3    1.25 A          1.18 A           166.7
 
   RHF/6-31G*   1.20 A          1.15 A           174.2 
 
Similar deviations from linearity are seen in the angles of substituted
isocyanates. What is the expected N-C-O angle based on experiment and/or
higher levels of theory? 

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Should be linear. I would suggest trying an H-N=C=O CASSCF calculation. A
CCSD(T) calculation should also be feasible with a basis set of this size on
an RS/6000 type of machine. This smells like artifactual symmetry breaking
to me. I find it extremely interesting that RHF/6-31G* gave a non=linear
N-C-O angle. Also the 6-31G* has been known to give result in bent
geometries for linear carbon clusters,( such as C4-C7). I would recommend
using the pVDZ basis set for exploring this aspect of the problem in a cost
efficient fashion.

If you do not have any access to Xray diffraction data of similar systems,
and are concerned about site modeling or solid state effects, I would
reccomend fixing the N-C-O angle to be linear.

-Galen F. Gawboy
Pinnacle Technology Resources, Inc.

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The only experimental evidence I know of for isocyanic acid, HNCO, is from
spectroscopic data:

Hocking, Gerry and Winnewisser, Can. J. Phys., 53, 1869 (1975)
Hocking, Gerry and Winnewisser, Astrophys. J., 174, L93 (1972)

as quoted by Harmony et al, J. Phys. Chem. Ref. Data, 8(3), 619 (1979).  I
don't have copies of the primary literature with me, but Harmony's comments
is that "it was necessary to assume linearity of NCO group.  The reported
values of NH and HNC are most strongly affected by this assumption, but the
heavy-atom distances are also affected to a smaller extent."

Values quoted:

NC      1.209A
CO      1.166
NH      0.986
HNC     128.0

Hope this is of some help to you.  Please summarise for CCL, as I'd be 
interested in seeing responses that you get.

Prof. Helder M. Marques, Department of Chemistry
University of the Witwatersrand

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I can't give you any experimental results on isocyanates, but I have
optimized HNCO (Cs symmetry) at various levels (see below).  The simple
answers to your last question are 1) HNCO is not linear at any level of
theory (including DFT and MP2), and 2) semiempirical methods do not "fail";
they actually reproduce other theoretical models reasonably well.  Here are
my results (they are close to, but not identical to what you reported).

Theory	         Energy		r(NC)		r(CO)	angle(NCO)
AM1		-15.15		1.232		1.202	166.7
PM3		-15.29		1.251		1.181	168.6
HF/6-31G*	-167.761384	1.200	        1.145	174.2
MP2/6-31G*	-168.232430	1.224	        1.184	171.7
HF/6-31G**	-167.766024	1.199	        1.148	174.3
MP2/6-31G**	-168.242279	1.225		1.183	171.7
B3LYP/6-31G**	-168.681498	1.218		1.174	172.2
BP86/6-31G**	-168.689231	1.230		1.186	171.3
SVWN/6-31G**	-167.794926	1.216		1.177	172.3

Comments on structures:
1. Basis set has little effect on HF and MP2 structures
2. HF bond distances are much shorter than MP2 (especially CO distance) and
bond angles are larger
3. Both PM3 and AM1 duplicate MP2 structure at least as well as HF
4. Agreement between MP2 and DFT is good for all types of DFT, but best
agreement is between BP86 and MP2
5. Bond angle at MP2 and DFT is approx 172.  The NCO unit is NOT linear.

Comments on calculations:
1. All calculations used Cs symmetry
2. AM1, PM3, HF, MP2 calculations performed with Spartan 4.0.3
3. MP2 optimizations done with "unfrozen" core
4. DFT calculations performed with Gaussian 94
5. Also tried SVWN/6-31G** with PS-GVB (fine mesh) and obtained essentially
identical result.

If I recall correctly, sometime back in the '80s (or possibly very late
'70s),HF Schaefer published an article in JACS on the structure of cumulenes
like 1,2,3-butatriene  (H2C=C=C=CH2) and found that these were not linear.
I don't have the reference, but you should be able to find it without too
much trouble. 
Good luck!

Alan Shusterman
Department of Chemistry
Reed College

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We have been doing some computational research involving isocyanates, and we
have found that, with the exception of silyl isocyanates, these cumulenes
are not linear. This result is in agreement with experimental data. The
sp-hybridized carbon atom does not yield a bond angle of 180 deg. because
the lone pair of the nitrogen atom interacts with the pi*(C=O)orbital. This
stabilizaing two-electron interaction is enhanced when the
O-C-N atoms are not linear. Therefore, the HF results, particularly those
obtained at HF/6-31G* level, reflect correctly this interaction.
	
Fernando Cossio

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I've performed _ab initio_ calculations on .alpha.-methylazo alkyl
isocyanates some years ago (RT Kroemer et al., J. Chem. Soc. Perkin-II
(1994) pp2129). These calculations were HF/6-31G* and MP2/6-31G*. The most
remarkable geometrical change at these levels of theory was a decrease in
the N-C-O bond angle from 171.5 to 167 degrees. This small angle can be
partially explained by a perturbation induced by the nearby diazene group.

However, as I was initially surprised by the fact that the N-C-O group is
not linear, I performed calcs on CH3-N=C=O and found a bond angle of 174.9
degrees at the HF/6-31G* level. 

The non-linearity and the decrease in the angle comparing HF/6-31G* and
MP2/6-31G* was also observed by other people (S Ellis et al., Can. J. Chem.
69 (1991) pp1000) for H2B-N=C=O and F2B-N=C=O.

If you perform a search for R-N=C=O in the Cambridge Crystallographic
Database, you will find at least 50 structures (either -N=C=O as a ligand in
metalloorganic compounds or as part of "real" organic compounds). In none of
these structures you will find absolute linearity, the N-C-O angle varying
>from approx. 165 to 178 degrees.

Of course one can argue that the levels of theory mentioned above are not
enough (one needs larger basis sets and more sophisticated schemes for
taking electron correlation into account than MP2), and in the crystal
structures the N-C-O angle is non-linear because of neighbour group effects
and crystal packing etc.

Therefore, it would be interesting for me to know where you found the
"observed linearity of the N-C-O" angle (maybe spectroscopy?).

Maybe you could try a really sophisticated calculation on H-N=C=O (such as
QCISD(TQ) or a coupled cluster calculation with a large basis set, the
system itself is not very large).

Dr. Romano T. Kroemer
Phys. & Theoret. Chem. Lab.
University of Oxford

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