7 Biologically Relevant Molecules Studied in Low Temperature Inert Matrices
195
very interestingly, it was recently demonstrated that, upon broadband near-IR/
mid-IR irradiation (with the IR source of a FtIR spectrometer) the population ratio
of the two amino-hydroxy conformers of cytosine changes towards a ratio corresponding to a photostationary state [82]. Evolution of the conformer population
ratio towards the photostationary ratio occurs independent of the initial ratio of
conformers, which could be prepared by a population shift (in favor of one of
the forms) induced by narrowband selective near-IR excitation (Fig. 7.18). In
addition, spontaneous tunneling conversion of the higher-energy Ah2 conformer into the lowest-energy Ah1 form was also observed for cytosine isolated in
a low-temperature argon matrix kept in the dark. this process was found to be
slow, occurring on a time scale of days, and, due to the inhomogeneous nature
of the matrix media, follows a dispersive type of kinetics rather than the classical
mono-exponential kinetics (Fig. 7.19) [82].
3630
3600
3570
0
40
80
0.0
0.1
0.2
0.3
1630
1610
0
200
400
600
0.0
0.1
0.2
1450
1420
0
200
400
0.00
0.03
0.06
810
800
0
30
60
0.00
0.02
0.04
7013 cm
-1
7034 cm
-1
km mol
-1
Absorbance
Absorbance
km mol
-1
Wavenumber / cm
-1
AH1
AH2
Fig. 7.17 Fragments of the IR spectra of cytosine isolated in an Ar matrix: ( red) after irradiation
at 7,013 cm
−1
; ( blue) after irradiation at 7,034 cm
−1
; compared to the (scaled) IR spectra calculated
for AH1 and AH2 conformers at the dFt(B3LYP)/6-31++g(d, p) level. (Reproduced from [78]
with permission from the PCCP owner Societies)