Observation of paramagnetic Raman optical activity of nitrogen dioxide

Article abstract of DOI:10.1002/anie.201403887

Raman optical activity (ROA) detects the intensity difference between right and left circularly polarized scattered light, and thus brings about enhanced information about the molecules under investigation. The difference is quite small and the technique is mostly constrained to the condensed phase. For NO₂ in the presence of a static magnetic field, however, the ROA signal with high ROA/Raman intensity ratio was observed. The signal is so strong owing to molecular paramagnetism and a pre-resonance signal enhancement. The spectral shape was explained on the basis of the Fermi golden rule and rotational wave functions expanded to a spherical top basis. The results indicate that the technique can be immediately used to obtain information about molecular properties, such as polarizability components. It also has a potential to detect other paramagnetic gases and discriminate among them. Paramagnetic gases: Paramagnetic Raman optical activity of nitrogen dioxide was observed for the first time and explained on the basis of angular momentum theory (see picture). The technique provides enhanced information about molecular properties, and the results suggest that the technique can be immediately used for discrimination and analysis of similar paramagnetic gases.

Full text of DOI:10.1002/anie.201403887

.
Angewandte
Communications
DOI: 10.1002/anie.201403887
Paramagnetic Raman Optical Activity
Observation of Paramagnetic Raman Optical Activity of Nitrogen
Dioxide**
ˇ
Jaroslav Sebestꢀk* and Petr Bou rˇ *
Abstract: Raman optical activity (ROA) detects the intensity
difference between right and left circularly polarized scattered
light, and thus brings about enhanced information about the
molecules under investigation. The difference is quite small
and the technique is mostly constrained to the condensed
Raman optical activity (ROA) is often more convenient
than infrared techniques as it covers a wider part of the
vibrational spectrum. It can be applied to small molecules as
well as to large biopolymers and even viruses, and it benefits
[
10]
of a flexible experimental setup. Typically, however, the
ratio of the ROA to Raman intensity (traditionally referred to
as CID, circular intensity difference) is very small, typically
phase. For NO in the presence of a static magnetic field,
2
however, the ROA signal with high ROA/Raman intensity ratio
was observed. The signal is so strong owing to molecular
paramagnetism and a pre-resonance signal enhancement. The
spectral shape was explained on the basis of the Fermi golden
rule and rotational wave functions expanded to a spherical top
basis. The results indicate that the technique can be immedi-
ately used to obtain information about molecular properties,
such as polarizability components. It also has a potential to
detect other paramagnetic gases and discriminate among them.
À4
about 10 . Increasing the ROA signal or CID, for example,
[11]
[12]
by surface enhanced scattering or induced resonance is
often problematic and not suitable to a general sample. The
ROA technique thus cannot be routinely applied to com-
pounds in the gas phase, although recently we could measure
[
13]
ROA of methyloxirane vapor.
Similarly, ROA of achiral samples kept in a static
magnetic field is traditionally restricted to the condensed
phase. It brought about useful information, for example,
about metal complexes such as the ferrocytochrome c chro-
C
hiral phenomena exploring different absorption or scatter-
[10a,14]
ing of left and right circularly polarized light constantly attract
attention because they provide enhanced information about
molecules, most typically in the form of spectral bands
differing in sign. Commonly used techniques include, for
example, optical rotation and optical rotatory dispersion,
mophore.
Most (diamagnetic) gases only provide an
immeasurably weak signal. However, the molecule of nitro-
gen dioxide enhances the measurement in two ways. First, it is
paramagnetic; the free electron lends the molecule a magnetic
moment, by several orders stronger than in the diamagnetic
case. (The ratio is approximately given by the Bohr and
[
1]
[2]
dating back to the early career of Luis Pasteur, electronic
and vibrational circular dichroism, or vibrational Raman
optical activity. The traditional techniques inspire new
methodologies, such as the cavity ring down polarimetry,
rotationally resolved vibrational circular dichroism, or time-
resolved experiments.
[
3]
À24
À1
nuclear magnetons, m ꢀ 9.274 ꢀ 10 JT vs. m = 5.051 ꢀ
B
N
[
4]
À27
À1
10 JT ). Second, many NO electronic levels pre-resonate
2
[5]
with the impinging laser radiation, so that the overall Raman
scattering and paramagnetic ROA (PROA) are very large.
[6]
[
7]
Needless to say, NO is one of the most common and most
2
It has been soon recognized that magnetic field must be
taken into account to explain such experiments. For chiral,
non-symmetrical molecules the magnetic component is pro-
vided by the motion of molecular electrons. But the magnetic
dangerous atmospheric pollutants. It is a commodity for
making a wide range of products including explosives,
fertilizers, and drugs. Atmospheric chemistry is affected by
[15]
UV absorption on NO2, which can be studied, for example,
[8]
[16]
field can also be imposed externally on an achiral sample,
giving rise to the family of magneto-optical phenomena, such
by monitoring nitrogen isotopic ratios.
The 1995 Nobel
Prize was awarded for the atmospheric chemistry of nitric
[
9]
[17]
as the Faraday effect or magnetic circular dichroism.
oxide and nitrogen dioxide involved in ozone depletion.
The toxicity, comprising a destruction of epithelial cells in the
lungs as the most immediate threat, has been documented in
[18]
many studies.
ˇ
*] Dr. J. Sebestꢀk, Prof. P. Bourˇ
[
By a chance, nitrogen dioxide has amazingly complex
spectra with a wealth of rotational, vibrational, and electronic
Department of Molecular Spectroscopy, Institute of Organic
Chemistry and Biochemistry, Academy of Sciences
Flemingovo nꢁm eˇ stꢀ 2, 166 10 Prague (Czech Republic)
E-mail: sebestik@uochb.cas.cz
[
19]
levels, often described as chaotic. Nevertheless, a large part
of its rotational, vibrational, and electronic quantum chemis-
bour@uochb.cas.cz
[20]
try has been elucidated, and the spectra presented in our
[
**] The work was supported by the Academy of Sciences (grant number
M200550902), Grant Agency of the Czech Republic (grant numbers
P208/11/0105 and 14-00431S), and Ministry of Education (grant
number LH11033). We thank Mr. Ond ˇr ej Pa cˇ es and Dr. Josef
Kapitꢁn for the help with the magnetic cell construction, and Dr.
Radek Pelc for discussions on the manuscript.
study are explicable on the basis of the angular momentum
theory.
[21]
We developed NO chemically, then purified the gas and
2
condensed it to a stock liquid. This proved convenient for
relatively lengthy measurements, and for minimizing the
effects of oxidation or laser light-induced decomposition. The
commercial Biotoolsꢁ ROA instrument based on the design of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403887.
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236
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tum numbers, J) quickly fades and becomes undetectable.
The Raman and PROA signal of the n2 fundamental
À1
vibration centered around 1320 cm is quite similar as for n1,
which can be understood because of the same symmetry of
these vibrations. On the other hand, the asymmetric N=O
À1
stretching vibration n around 1616 cm provides a relatively
3
weak Raman intensity, almost invisible at this scale, both in
experiment and in theory. Interestingly, it does yield meas-
urable ROA peaks (of “w”-shape, with extremes at 1603, 1625
and 1639 cm , simulated at 1620, 1633, and 1644 cm ,
respectively).
Figure 1. Geometry of the PROA experiment.
À1
À1
[22]
W. Hug and G. Hangartner was expanded by the addition of
a magnetic cell made of four neodymium magnets, providing
a field of 1.5 tesla in the cell compartment. In the scattered
circular polarized (SCP) backscattering experiment
The most complex ROA signal appears within the 1400-
À1
1550 cm interval. One can identify the combination band
À1
2n at about 1500 cm , where several rotational transitions
1
(
Figure 1) the sample was irradiated by linearly polarized
can be assigned to the modeled Raman and ROA spectra.
light, and the difference of the right and left circularly
However, the relatively strong experimental Raman bands at
R
L
À1
polarized scattered light (I ÀI ) was detected.
Selected portions of ROA and Raman spectra are
presented in Figure 2. The entire spectra can be found in
the Supporting Information
1518 and 1546 cm seem to disturb the 2n signal, and most
2
likely can be attributed to a fluorescence or coupling of
electronic and vibrational levels (cf. also Figure S3). Finally,
(
Figure S2),
together
with
experimental CID ratio and
the degree of circularity, DOC.
[
23]
As pointed out previously,
depending on experimental con-
ditions, pre-resonance Raman
vibrational spectrum overlaps
with fluorescence; occasionally
the fluorescence may dominate.
In our case, however, the vibra-
tional components were much
larger. The signal of the first
fundamental mode n , upper-left
1
panel in Figure 2) is probably
the most pronounced. Its
Raman intensity is concentrated
in the central “Q” band (around
À1
7
50 cm ). Satellite rotational
bands can be recognized as
well, and modeled with reason-
able accuracy even at the lim-
ited resolution and sensitivity of
the single-monochromator spec-
trometer. Examples of the
strongest Raman transitions
contributing to the spectra are
listed in Table S3 of SI. The
ROA spectrum is dominated
by a + /- “couplet” (close oppo-
site bands) measured at 735/
À1
7
7
47 cm and calculated at 746/
À1
53 cm . It is followed by
another couplet, experimentally
À1
at 778/782 cm , calculated at
À1
7
77/783 cm . The intensity of
rotational bands further away
from the Q-band (for transitions
involving large rotational quan-
R
L
R
L
Figure 2. Experimental (blue) and simulated (black) PROA (I ÀI ) and Raman (I +I ) NO spectra of the
2
five vibrational transitions (n , n , n , 2n , n +n ). The normal mode displacement for the three
1
2
3
2
1
2
fundamentals is indicated as calculated at the B3LYP/6-311+ +G** level of approximation.
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Angewandte
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À1
for the “isolated” n + n combination transition at 2070 cm ,
1
2
the Raman and ROA experimental spectra again appear to be
fully explicable by the simulation, although the signal is
relatively weak and more strongly affected by experimental
noise than that of the n and n bands, for example. Still, the
1
2
intensities of both the 2n and n + n combination bands are
2
1
2
remarkably strong, thus confirming a significant deviation of
the vibrational potential of this molecule from the harmonic
[20d]
energy landscape.
The spectral shapes reflect contributions of individual
polarizability components, as documented for the n signal in
1
[
8,24]
Figure 3. The transition polarizability
of this vibration is
diagonal, with three non-zero components. As apparent from
the figure, they generate very specific PROA spectral patterns
that are easier to distinguish, at least by visual comparison,
than the Raman intensity patterns. Note that the observed
spectrum is not a simple sum of the three contributions.
However, further improvement of experimental sensitivity is
desirable for a more accurate experimental verification of the
polarizability used in the modeling.
Figure 4. ROA and Raman spectra of pure NO (blue) and the NO
mixture (red).
2
x
resolution and noise. The possibility to relate the spectra to
molecular polarizability, and the generally higher sensitivity
of PROA to the system chemistry indicate a tremendous
potential of the PROA spectroscopy in analytical chemistry of
the nitric dioxide and other paramagnetic gases.
Received: April 1, 2014
Published online: July 2, 2014
Keywords: angular momentum theory · nitrogen dioxide ·
.
paramagnetic gases · Raman optical activity ·
spectral simulations
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