The effect of an anci
llary water mo
lecu
le on the neutra
l anda
lka
line hydro
ly
si
s mechani
sm
s of a
simp
le
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le">-
lactam mo
lecu
le (
N-methy
lazetidinone) ha
s been
studied at the Hartree-Fock and MP2
leve
ls u
singthe 6-31G* and 6-31+G* ba
si
s set
s. The re
su
lt
s have beencompared with a nona
ssi
sted
study carried outpreviou
sly. So
lvent effect
s have been a
lso con
sidered by mean
s ofa po
larizab
le continuum mode
l. In theneutra
l hydro
ly
si
s, the additiona
l water mo
lecu
le dimini
she
s thefree-energy barrier
s on
ly when corre
lationenergy i
s taken into account. Concerted and
stepwi
se mechani
sm
shave been de
scribed. The corre
spondingbarrier
s are c
lo
se, and the actua
l mechani
sm cou
ld be conditioned bythe mo
lecu
lar environment,
so
lution,protein, etc. U
sing the re
su
lt
s of a mo
lecu
lar dynamic
s simu
lationof
N-methy
lazetidinone in aqueou
s so
lution,it ha
s been
shown that the
stepwi
se proce
ss i
s more
like
ly to occur in
such condition
s. In the a
lka
line hydro
ly
si
s,the fir
st reaction
step con
si
st
s of the formation of a tetrahedra
lintermediate which require
s a de
so
lvation ofthe hydroxy
l anion difficu
lt to reproduce by ca
lcu
lation.Afterward, the hydro
ly
si
s reaction proceed
s througheither concerted or
stepwi
se mechani
sm
s for ring opening and protontran
sfer. The concerted channe
l pre
sent
sa very
low energy barrier, and the
specie
s invo
lved are dependent onthe ca
lcu
lation
leve
l. The
stepwi
semechani
sm i
s virtua
lly the
same a
s that previou
sly reported for thenona
ssi
sted hydro
ly
si
s, the re
lative energyof a
ll the point
s a
long the path being dimini
shed and the energybarrier
s remaining e
ssentia
lly una
ltered.