From: Sundeep Rayat 
Reply-To: CH412F99-L@po.missouri.edu
To: CH412F99-L@po.missouri.edu, ch412f99-l@lists.missouri.edu
Subject: POTW-club Sundeep-Stability of Cyclic Acetoxy-N-nitrosamines
Date: Thu, 16 Sep 1999 00:06:06 -0500
MIME-Version: 1.0

 Hello! Everybody, Here is my contribution for the "Paper of the week"..


Title: "An ab Initio Quantum-Mechanical Study of the Stability of Cyclic
alpha Acetoxy - N-nitrosamines:Amine N-to-NO Dative Bonding in alpha
Hydroxy - N-nitrosamines versus N-to-Carbocation Dative Bonding in
N-nitrosiminium Ions." 

Author: Rainer Glaser 

Reference: J. Am. Chem. Soc. 1999, 121, 5170-5175.  

Goals: The goal of the paper is to provide an explanation for the
difference in the energies of the unimolecular dissociation of
a-hydroxy-N-nitrosopyrrolidine 1 and a-hydroxy-N-nitrosopiperidine 2 to
their respective a-hydroxy-N-nitrosiminium ions. 

Objectives: The objective of the paper is to explain the differences in
the ionization of the two cyclic nitrosamines 1 and 2 in terms of strong
N->NO vs N->Carbocation dative bonding. To reach the goal, the
transition states for the rotation around the NN bond have been located
for all the four structures 1, 2, 3 and 4 and the activation barriers
for rotation has been determined and  taken as a direct measure of the
strength of N->NO dative bond. Furthermore, charges on different atoms
and different fragments of these compounds have been calculated that
indicates the flow of electron density from NO to the developing
carbocation center or in otherwords s -polarizations occur in the
ionization process.  

Methods: Structures optimized at RHF 6-31G*, Single point energy
calculated at MP2, MP3, and MP4(SDTQ). The charged were calculated using
NBO, CHELPG, MKS. 

Discussion: The optimization of the structures 1, 2, 3 and 4 indicate
that the ionization of 1 to 3 or of 2 to 4 causes the elongation of N-N
bond and the shortening of the C-N bond. The extent of shortening of C-N
bond is greater than the elongation of N-N bond in the ionization of
both the compounds 1 and 2 to 3 and 4 respectively. But this effect is
more pronounced in five membered nitrosamines i.e. in 1. The shortening
of C-N bond and elongation of N-N bond is due to the existence of dative
bonding as indicated in the resonance structures in Scheme 2. Also the
dissociation of 1 was found to be endothermic than the 2 or in
otherwords nitrosiminium ion 4 is formed preferentially compared to 3.
To provide an explanation for this, the rotational barriers for 1, 2, 3
and 4 has been calculated. It was found that it is more difficult to
rotate the NN bond(by about 6.3 Kcal/mole) in  1 than in case of 2. This
is as a result of enhanced delocalization of the N-lone pair in 1 in the
Transition state. This delocalization relieves the ring strain
associated with the five membered ring 1 by changing the hybridization
of ring Nitrogen from sp3 to sp2. This indicates a strong N->NO bond in
1 which makes it more stable.  The rotational barriers in  1-TS to 4-TS
have been used to consider the ionization in the absence of the N->NO
bond which shows that the N->NO is unimportant than the N->carbocation
dative bonding. 
The electron structure analysis studies indicate ionization to
nitrosimines causes the flow of electron density from the NO and this
effect is more pronounced in case of 1. 

I chose this paper because it is easy to understand and interesting in
terms of reading. It kept me focused the entire time, when I first read
it. The good thing about this paper is this that it is written in an
easily understandable language and one does not have to have a strong
theoretical background to read it as opposed to some other papers. 

ALthough it didn't understand the explanantion for the structural
relaxation in the presence and absence of N->NO p dative bonding.   

Thanks, 
Sundeep.
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