Photoinduced fluorescent cross-linking of 5-chloro- and 5-fluoro-4-thiouridines with thymidine

Article abstract of DOI:10.1021/jo902136k

(Chemical Equation Presented) Two highly fluorescent, thermally stable diastereomeric photoadducts, 3a,b, are formed when either 5-chloro-4- thiouridine, 1, or 5-fluoro-4-thiouridine, 2, are photoexcited with 366 nm UV light in the presence of thymidine (T). 5-Fluoro-4-thiouridine, 2, exhibits photoreactivity much higher than that of the 5-chloro derivative 1. In both cases the photoreaction is very clean, leading to highly eficient conversion of the 5-halogeno-4-thiouridine (1, 2) and T to photoadducts 3a,b. The identity and structure of 3a was confirmed using mass spectrometry and 2-D NMR. The epimeric relationship of 3a,b was established by UV circular dichroism spectroscopy. The geometry of the fluorescent photoadduct is consistent with formation of an interstrand cross-link in a DNA duplex if 1 or 2 is flanked by T in an opposite strand.

Full text of DOI:10.1021/jo902136k

pubs.acs.org/joc
Photoinduced Fluorescent Cross-Linking of 5-Chloro- and
5-Fluoro-4-thiouridines with Thymidine
,†
Bohdan Skalski,* Katarzyna Taras-Goslinska, Anna Dembska, Zofia Gdaniec, and
†
†
‡
ꢀ ꢀ
Stefan Franzen§
†
‡
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland, Institute of
ꢀ
Bioorganic Chemistry, Polish Academy of Sciences, Z. Noskowskiego 12/14, 61-704 Poznanꢀ, Poland, and
§Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695
bskalski@amu.edu.pl
Received October 8, 2009
Two highly fluorescent, thermally stable diastereomeric photoadducts, 3a,b, are formed when either
5-chloro-4-thiouridine, 1, or 5-fluoro-4-thiouridine, 2, are photoexcited with 366 nm UV light in the
presence of thymidine (T). 5-Fluoro-4-thiouridine, 2, exhibits photoreactivity much higher than that
of the 5-chloro derivative 1. In both cases the photoreaction is very clean, leading to highly eficient
conversion of the 5-halogeno-4-thiouridine (1, 2) and T to photoadducts 3a,b. The identity and
structure of 3a was confirmed using mass spectrometry and 2-D NMR. The epimeric relationship of
3a,b was established by UV circular dichroism spectroscopy. The geometry of the fluorescent
photoadduct is consistent with formation of an interstrand cross-link in a DNA duplex if 1 or 2 is
flanked by T in an opposite strand.
The 4-thiouracil (4SU) chromophore and its derivatives
have attracted considerable interest in recent years as effi-
cient photo-cross-linking agents in the studies of the struc-
ture of nucleic acids and their interactions with proteins.1-3
The use of 4-thiouridine and 4-thiothymidine as structural
probes2,4 indicates that a range of cross-links is possible,
including interstrand cross-links.5 To identify the covalent
bonds that form and to better understand the cross-linking
mechanism, a number of experiments involving photoirra-
diation of thiouridine have been conducted in solution in the
presence of common nucleosides or using dinucleotides.6-9
In agreement with the results of 4SU photo-cross-linking
experiments with DNA, it has been demonstrated that 4SU
forms mixed photoadducts with all nucleobases with thymi-
dine being the best acceptor.8,9 The (5-4) or (6-4) bipyrimi-
dine compounds were identified as major photoproducts of
irradiation of 4SU in the presence of pyrimidine nucleosides.
(1) Favre, A. In Bioorganic Photochemistry; Morrison, H., Ed.; J. Wiley &
Sons: New York, 1990; Vol. I, pp 379-425.
(6) Bergstrom, D. E.; Leonard, N. J. Biochem. 1972, 11, 1–9.
(7) Blazek, E. R.; Alderfer, J. L.; Tabaczynski, W. A.; Stamoudis, V. C.;
Churchill, M. E.; Peak, J. G.; Peak, M. J. Photochem. Photobiol. 1993, 57,
255–265.
(8) Favre, A.; Dubreuil, Y. L.; Fourrey, J. L. New J. Chem. 1991, 15, 593–
599.
(2) Favre, A.; Saintome, C.; Fourrey, J. L.; Clivio, P.; Laugaa, P.
J. Photochem. Photobiol. B 1998, 42, 109–124.
(3) Meisenheimer, K. M.; Koch, T. H. Crit. Rev. Biochem. Mol. Biol.
1997, 32, 101–140.
(4) Favre, A.; Fourrey, J. L. Acc. Chem. Res. 1995, 28, 375–382.
(5) Saintome, C.; Clivio, P.; Favre, A.; Fourrey, J. L.; Laugaa, P. Chem.
Commun. 1997, 167–168.
(9) Peak, M. J.; Midden, W. R.; Babasick, D. M.; Haugen, D. A.
Photochem. Photobiol. 1988, 48, 229–232.
DOI: 10.1021/jo902136k
r
Published on Web 01/05/2010
J. Org. Chem. 2010, 75, 621–626 621
2010 American Chemical Society

Skalski et al.
JOCArticle
Non-native 5-halogenated pyrimidines have been studied in
parallel to the 4-thiopyrimidines as probes of nucleoprotein
interactions as well as DNA structure.3
In a search for new photoprobes, we have synthesized a
series of combined 5-halogeno-4-thiouridines (5X-4SU; X =
F, Cl, Br, I).10-12 On the basis of spectroscopic and structural
studies, wehave recently discovered two efficiently fluorescent
photoproducts that resulted from a photochemical reaction of
either 5Cl-4SU (1) or 5F-4SU (2) with thymidine (T) as shown
in Scheme 1. The two isomeric photoproducts 3a,b formed in
the photochemical reaction with T were the same for both 1
and 2. Neither of these highly fluorescent photoproducts was
observed for reactions of 5Br-4SU or 5I-4SU with T.
SCHEME 1. Photoreaction of 5-Halogeno-4-thiouridines 1,2
with Thymidine, T (λex = 366 nm)
FIGURE 1. Changes in the absorption spectra of the solution of 1
and T observed during irradiation by 366 nm UV light for 75 min.
The absorption band centered at 270 nm is due to T mainly, whereas
that at 340 nm due to 1. Inset: comparison of the rates of disap-
pearance of 1 and 2 upon irradiation at 366 nm. The time constants
given are 1/e times.
The isomeric photoproducts 3a,b both have high fluores-
cence quantum yields. Moreover, our preliminary experi-
ments indicate that analogous fluorescent adducts are
formed in RNA:DNA and DNA:DNA hybrids having
5F-4SU incorporated in one of the strands, leading to highly
efficient interstrand photo-cross-linking of the complemen-
tary oligonucleotides. Given the widespread use of fluores-
cence, the novel photoproduct we report has many potential
applications. Herein, we describe the spectroscopy and
structure of the two isomeric products formed when the
5-chloro-4-thiouridine or 5-fluoro-4-thiouridine are photo-
excited with 366 nm light in aqueous solution in the presence
of thymidine.
Figure 1 shows the changes in the absorption spectra of
both 1 and T upon selective excitation of 1 at 366 nm for
75 min. The downward arrows in the figure indicate the
decrease in absorption intensity of the starting materials, 1
and T, at 340 and 270 nm, respectively. The upward arrows
at 242-246 nm and above 370 nm correspond to the
absorption bands of the photoproducts formed. HPLC
analysis of the irradiated solution reveals that both 1 and T
are consumed and two photoproducts are formed. Similar
spectral changes and formation of the same two photopro-
ducts (cf. Figure 2) occur for the irradiation of 2 and T but
with a ∼4-fold higher rate (cf. Figure 1, Inset). This reflects
much higher photoreactivity of the 5-fluoro derivative toward
T and correlates well with the ratio of the measured quantum
yields for the disappearance of both compounds in the
FIGURE 2. Comparison of the HPLC analyses for the preparative
scale irradiations (λex > 300 nm) of 1 (upper part) and 2 (lower part)
in the presence of T (2 equiv). The solid lines correspond to the
mixtures before irradiation, and dotted lines corespond to the
mixtures irradiated to ca. 90% conversion of 1 and 2 (see Support-
ing Information for the full scale HPLC absorption and fluores-
cence elution profiles and respective UV spectral data). Careful
analysis of the HPLC data reveals that the photoadducts 3a and 3b
are formed with the ratios of 1:1.9 and 1:1 and overall yields of 72%
and 92% for 1 and 2, respectively.
(10) Taras-Goslinska, K.; Wenska, G.; Skalski, B.; Maciejewski, A.;
Burdzinski, G.; Karolczak, J. J. Photochem. Photobiol. A 2004, 168, 227–233.
(11) Wenska, G.; Taras-Goslinska, K.; Lamparska-Kupsik, K.; Skalski,
B.; Gdaniec, M.; Gdaniec, Z. J. Chem. Soc., Perkin Trans. 1 2002, 53–57.
(12) Wenska, G.;Taras-Goslinska, K.; Skalski, B.; Hug, G. L.; Carmichael,
I.; Marciniak, B. J. Org. Chem. 2005, 70, 982–988.
presence of T (Φ = 2.8 ꢀ 10-3 and 6.1 ꢀ 10-4 for 2 and 1,
respectively).
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FIGURE 4. Structure of 7-(3,4-dihydroxy-5-hydroxymethyl-tetra-
hydro-furan-2-yl)-1-(4-hydroxy-5-hydroxy-methyl-tetrahydro-furan-
2-yl)-4a-methyl-4a,9a-dihydro-1H,7H-9-thia-1,3,5,7-tetraaza-fluor-
ene-2,4,6-trione, 3a, confirmed by NMR spectroscopy. The arrows
show the connectivity of the crosspeaks due to coupling through
2- and 3-carbon atoms observed in the HMBC experiment.
protons, respectively. We will refer to the sugar atoms as
deoxy Cn0 or ribo Cn0, respectively, where n represents the
standard numbering.
FIGURE 3. Comparison of the normalized absorption and fluor-
escence emission spectra of the isomers 3a and 3b.
The NMR data were used to establish the identity of the
photoproducts as two diastereomers. From the four theore-
tically possible diastereomers two would be trans-fused at
the C4a-C9a bond and thus unlikely. In both the possible
cis-fused diastereomers 4a-CH3 and 9a-H are cis oriented.
These two diastereomers thus form a pair of C4a,C9a-
epimers. On the basis of the integration we conclude that
the 1.69 ppm peak corresponds to the methyl at position C4a.
Using NOEs we determined that the methyl group is cis to
the 9a-H at 5.91 ppm in each of the isomers. The NOE
observed between the ribo 10-H (5.94 ppm) and the peak at
8.30 ppm permits assignment of this peak as the 8-H of the
9-thiafluorene ring. The detailed proof of the structure is
presented for isomer 3a, which is shown in Figure 4. The
proof for isomer 3b is completely analogous.
TABLE 1. Measured Absorption and Fluorescence Parameters for
Isomers 3a and 3b
absorption
fluorescence
isomer
λmax (nm)
ε (M-1 cm-1
)
λ
max (nm)
φ
τ (ns)
3a
244
372
242
367
12 800
3 700
13 700
3 800
470
460
0.49
14.2
3b
0.26
7.3
Figure 3 shows the normalized absorption and fluores-
cence spectra of the two fractions corresponding to the two
photoproducts obtained from HPLC analysis. Both absorp-
tion and fluorescence spectra of species 3a and 3b are very
similar; however, there are systematic differences as indi-
cated in Table 1. Excitation at 360 nm produces emission at
470 nm with a quantum yield of 0.49 and 0.26 for species 3a
and 3b, respectively. The fluorescence lifetimes of the isomers
3a and 3b were measured to be 14.2 and 7.3 ns, respectively.
Although we do not have an explanation for the difference in
quantum yield at this time, analysis of the observed lifetimes
and quantum yields shows that 3a and 3b have nearly
identical radiative lifetimes of 29 and 28 ns, respectively.
Thus, the difference in their quantum yields is due to non-
radiative processes, which are not included in any of the
calculations presented here.
High-resolution mass spectrometry (see Supporting
Information) showed that both photoproducts, 3a and 3b,
have the same molecular composition of C19H24N4O10S,
which indicates the formation of two isomers. The two
isomers were formed irrespective of identity of the 5-halogen
atom. The same photoaddition products were formed from
the reaction of either 1 or 2 and T with a loss of HCl or HF,
respectively.
All 19 carbon signals can be identified in the 13C NMR
spectrum in agreement with the molecular formula. Ten 13C
resonances are assigned to the sugar residues (vide supra), and
these are given in the Supporting Information. There are six
quaternary carbon atoms, one methyl group at 17.17 ppm,
and two carbon atoms at 135.88 ppm and 64.88 ppm that are
directly bonded to protons at 8.30 ppm (H8) and 5.91 ppm
(H9a), respectively.
The fluorene-like ring structure was established by analyz-
1
ing long-range proton-carbon correlations in the H-13C
HMBC spectrum. Specifically, the interconnection between
two rings could be established using the correlation of
carbon and hydrogen via J(CH) over two and three bonds.
The proton signal at 5.94 ppm attributed to ribo H10
correlates with signals at 155.91 ppm (C6) and 135.88 ppm
1
(C8) in the H-13C HMBC spectrum. The deoxy H10
exhibits a weak NOE to H9a at 5.91 ppm and shows
correlations with carbon resonances at 64.88 ppm (C9a)
and 152.21 ppm (C2) in the 1H-13C HMBC spectrum.
Crosspeaks between proton H9a at 5.91 ppm and carbon
atoms at 55.48 ppm (C4a), 115.38 ppm (C8a), and 174.56
ppm (C4b) establish the connectivity proposed in Figure 4.
Further evidence can be in the observed correlations between
the methyl protons and carbon atom at 174.56 ppm (C4b).
To further support the structure of the photoproduct, we
built two models. The model with C9a-S-C8a connectivity is
shown in Figure 4. A second possible model would have a
C4a-S-C4b connectivity, using the numbering of the mole-
cule in Figure 4. Analysis of these two models indicated that
The structure of both of the isomers was solved using
1
NMR methods. The H and 13C NMR spectra reveal that
two sugar moieties are present in the structure. The 1H and
13C signals were unambiguously assigned to ribose and
deoxyribose on the basis of 1H-1H COSY and 1H-13C
HSQC spectra (see Supporting Information). Apart from
the sugar protons there are only three singlets at 8.30, 5.91,
1
and 1.69 ppm in the H NMR spectrum. These resonances
had integrated intensities corresponding to 1, 1, and 3
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Skalski et al.
JOCArticle
only the proposed structure with C9a-S-C8a, shown in
Figure 4, is possible. In the model with the C4a-S-C4b
connectivity, a close contact between proton H9a and H8
would have been detected in NOE-type spectra and the
strong cross peak between the methyl protons and C4b
would have been absent in 1H-13C HMBC spectrum.
The epimeric relationship between 3a and 3b was con-
firmed further by their CD spectra, which are presented in
Figure 5. The CD spectra indicate that the fused heterocyclic
FIGURE 6. Two possible isomers obtained from calculations are
indicated. (A) The thietane-F-3a intermediate structure is shown
along with the final rearranged product, 3a, that is formed when HF
is eliminated. (B) The corresponding intermediate and final struc-
ture for isomer 3b are shown.
C5-halogenation and C4-thionation of uridine provides both
the necessary leaving group, X = F- or Cl-, and nucleo-
phile, S, to promote the elimination required to form the
stable tetraza fluorene product.
It was not possible to experimentally observe the thietane-
X-3a,b intermediate that would verify the proposed mecha-
nism; however, the mechanism is strongly supported by DFT
calculations. The calculated structures of 3a,b along with the
corresponding thietane-F-3a,b intermediates are shown in
Figure 6, whereas dissociation and rearrangement energies
for the halogenated thietane-X series (X = H, F, Cl, Br, I)
are presented in Figure 7.
The reactivity of the modified bases with respect to
rearrangement of the thietane-X intermediate and loss of
HX is in agreement with theory. Figure 7A gives the calcu-
lated bond energies of H-X (see also Supporting Information
for values). The overall energies for the rearrangement of the
thietane intermediate to form the fluorocrosslink product,
3a,b is given by:
FIGURE 5. Circular dichroism spectra of isomers 3a and 3b.
and thus chromophoric parts of these compounds have
opposite configurations, and the diastereomerism results
from the presence of the two ribose residues in the structure.
The photoadducts 3a,b are structurally distinct from a 5-4
pyrimidine-pyrimidone photoadduct formed by irradiation
of 4SU in the presence of T6,7 as well as similar 5-4 and/or 6-4
bipyrimidine products formed upon irradiation of 4SU in the
presence of other pyrimidine bases.2 Furthermore, the spe-
cificity and the overall yield of 3a and 3b are significantly
larger than for photoadducts of 4SU with pyrimidine nucleo-
bases. It is generally accepted that the formation of 4SU
photoadducts occurs via thermally unstable thietane inter-
mediates formed upon addition of the thiocarbonyl group of
4SU to the C5dC6 double bond of another pyrimidine.
Thus, it is reasonable to assume that a similar mechanism
involving the initial formation of a fluorinated or chlorinated
thietane (also referred to as thietane-F and -Cl, respectively)
intermediate operates also in the case of 1 and 2 (Scheme 2).
thietane-X-3a; b f 3a; b þ HX
This reaction energy is shown in Figure 7B,C for 3a and 3b,
respectively (see also Supporting Information). The calcula-
tions reveal that the difference in the C-X and H-X bond
energies is the driving force for the rearrangement of the
thietane-X-3a,b intermediate. Although the C-F bond is
commonly regarded as the strongest bond in organic chem-
istry, the loss of HF can occur if there is an internal strain
such as exists in the thietane-F-3a,b intermediate. The calcu-
lation shows that loss of H2 is not favorable under these
conditions. Both theory and experiment show that loss of
HCl can also lead to formation of the fluorocrosslink
product. However, as the C-X bond weakens, e.g., for
C-Br and C-I, other bond breaking events can lead to
other competing reactions as shown by the extensive photo-
chemical studies of C-X bonds in the 5-halogenouridine
excited states.
SCHEME 2. Proposed Mechanism for Formation of 3a,b
The fluorinated thietane intermediate has the largest
driving force for rearrangement to form the photoproduct.
Consequently, this molecule has the greatest yield and most
favorable kinetics for formation as seen experimentally in
Figure 1. The chlorinated thietane intermediate is somewhat
less reactive. The brominated thietane intermediate is not
predicted to rearrange to form 3a,b, in agreement with the
observation that irradiation of 5Br-4SU and T does not
However, whereas in the case of the 4SU thietane-X inter-
mediate (X = H) H2S is eliminated prior the formation of a
stable photoproduct, 1 and 2 undergo, when X = Cl or F, a
ring opening followed by ring closure with loss of H-Cl or
H-F to form the tricyclic structures of 3a and 3b as outlined
in Scheme 2. Thus, it appears that the combination of
624 J. Org. Chem. Vol. 75, No. 3, 2010

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FIGURE 8. Calculated absorption and fluorescence spectra for a
model of 3a, which lacks deoxyribose and ribose sugars. The
vibrational frequencies and excited state displacements were calcu-
lated using DFT methods.
has also been found to be a candidate for clinical therapeutic
applications.19 The high specificity and yield of the cross-link
formed between 5F-4SU/5Cl-4SU and T presents new possibi-
lities in each of these realms of application. The geometry
formed in these solution studies is consistent with applications
in DNA heteroduplexes using a 5F-4SU/5Cl-4SU modified
nucleotide in an opposite flanking position to T. The creation of
a fluorescent crosslink under these conditions has potential
applications in both in vitro and in vivo identification of DNA
sequences.
FIGURE 7. Dissociation and rearrangement energies for reactions.
(A) The dissociation energies for the H-X (X = H, F, Cl, Br, I) series
is presented. (B) The rearrangement energy from the thietane-X
intermediate with atom X is presented for isomer 3a. (C) The
rearrangement energy is presented for isomer 3b.
Experimental Section
Synthesis of 1 and 2. 20,30,50-Tri-O-acetyl-5-halogeno-4-
thiouridines (X = F, Cl) were prepared as previously des-
cribed11 and subjected to de-O-acetylation with 2.5% ammo-
nium hydroxide in methanol/water mixture (1:1, v:v) to give the
corresponding unprotected nucleosides 1 and 2.
Photoreactions of 1 and 2 with T. For analytical scale experi-
ments aqueous solutions of 1 or 2 (0.2 mmol) containing an
equimolar amount of thymidine were placed in a 1 cm ꢀ 1 cm
UV cell, deoxygenated by bubbling argon, and irradiated on an
optical bench using a 200 W high-pressure mercury lamp
equipped with an interference filter to isolate the 366 nm line.
The progress of the reaction was monitored by UV spectro-
scopy and HPLC. The quantum yields for the disappearance of
1 and 2 were measured using benzophenone/benzhydrol actino-
metry.20
Preparative Irradiation for Product Analysis. The solutions of
1 or 2 (400 mL, 0.2 mmol) with T (2 equiv) were irradiated, in
portions, in a 80 mL photoreactor with a 150 W high pressure
mercury lamp through a Pyrex filter under an argon atmo-
sphere. Irradiation was continued to ca. 95% conversion of the
substrate as checked by HPLC. Irradiated solutions were col-
lected and concentrated under reduced pressure. The photo-
products were isolated from the residue by preparative HPLC
and identified by means of high resolution mass spectrometry
and NMR data as discussed above.
result in a fluorescent photoproduct. The case of iodine, 5I-
4SU is not comparable since loss of I by photolysis is facile.
12,13 The absorption and fluorescence spectra for the model of
3a, in which the sugar residue was replaced by a methyl group,
were also calculated (Figure 8) and are in agreement with the
measured spectra of 3a shown in Figure 3.
In summary, we have studied the formation of the photo-
product of 5X-4SU and T and shown that for X = F and Cl
there is a unique pair of isomeric photoproducts that form. The
photoproducts are fluorophores with higher fluorescence yield
than any known pyrimidine-pyrimidine adducts. The reaction
does not occur for X = H, Br, or I in agreement with cal-
culations. Thiouridine and thiothymidine have a wide range of
applications including template-directed photoligation,14
photoaffinity probes,3,15-17 and therapeutics.18 The 5Br-4SU
(13) Kawai, K.; Saito, I.; Sugiyama, H. Tetrahedron Lett. 1999, 40, 5721–
5724.
(14) Liu, J. Q.; Taylor, J. S. Nucleic Acids Res. 1998, 26, 3300–3304.
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Tetrahedron Lett. 1994, 35, 873–876.
Computational Methods. The optimized ground state geome-
tries and potential energy surfaces Thietane-X-1,2 and diaster-
eomers 3a,b were obtained using the GGA functional21
(17) Saintome, C.; Clivio, P.; Fourrey, J. L.; Woisard, A.; Laugaa, P.;
Favre, A. Tetrahedron 2000, 56, 1197–1206.
(18) Parker, W. B.;Shaddix,S. C.; Rose, L. M.;Tiwari,K. N.;Montogmery,
J. A.; Secrist, J. A. III; Bennett, L. L., Jr. Biochem. Pharmacol. 1995, 50, 687–
695.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993; p 307.
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J. Org. Chem. Vol. 75, No. 3, 2010 625

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as implemented in DMol3 (Molecular Simulations Inc.).22
Calculations were carried at the North Carolina State Univer-
sity High Performance Computing Center. Geometry optimiza-
tions were carried out without constraints until the energy
difference was less than 10-6 au on subsequent iterations.
Numerically tabulated basis sets of double-ζ plus polarization
(DNP) quality were employed. The difference between the
minima of the ground and excited states was employed to
estimate the magnitude of the nuclear displacement along each
normal mode. The calculated nuclear displacements, Δi, provide
the electron-phonon coupling constants, Si = Δi2/2 for the
corresponding Franck-Condon active mode of frequency ωi,
which along with temperature provides the necessary inputs for
the two-time formalism. For each normal mode, the vibration
displaces each respective atom constituting the structure in
Cartesian space by a magnitude, Δxi, Δyi, and Δzi. A dimension-
less nuclear displacement, Δ, was then calculated for each
displaced atom in the optimized structure. In order to verify
that the modes contained a single minimum, the nuclear dis-
placements, Q, were calculated for 11 geometries ranging from 1
au to -1 au (Qj = -1, -4/5, ..., 0, ..., þ4/5, þ1). The displaced
structure, representing the excited state for each respective
mode, was found by taking the geometry optimized Cartesian
coordinate for each respective atom and subtracting the mass
weighted normal mode eigenvector for each respective displace-
ment step:
!
ꢀ
ꢁ
Δqj
Mw1=2
Qj
Cj ¼
0:529167
qi = xi, yi, and zi, C is the new x, y, and z coordinate, and Mw is
the molecular weight of the corresponding atom. The factor of
0.529167 is the Bohr radius used to convert from atomic units to
˚
A. The dimensionless displacements along the ith normal mode,
Δi, was calculated by dividing the normal mode displacement by
1/Ri1/2
, where Ri = 2πμiωi/h. The conversion factor
(2π2mpc)/(1020 h) was employed to convert the energy to
cm-1. The time-correlator implemented in the program TIME-
THERM, developed by Shreve,23 was used to calculate absorp-
tion spectra at 300 K. The calculation also gave the Stokes shift
permitting calculation of the fluorescence spectrum.
Acknowledgment. S.F. and B.S. gratefully acknowledge
support from NSF grant OISE 0651876. Partial support from
the Ministry of Science and Higher Education (MNiSW)
Poland, project N N204 215434 is also acknowledged.
Supporting Information Available: Mass spectrometric
data, NMR data, output from DFT calculations and HLPC
analysis. This material is available free of charge via the Internet
at http://pubs.acs.org.
(22) Delley, B. J. Chem. Phys. 2000, 113, 7756–7764.
(23) Shreve, A. P.; Mathies, R. A. J. Phys. Chem. 1995, 99, 7285–7299.
626 J. Org. Chem. Vol. 75, No. 3, 2010