Molecular and electronic structure of δ-valerothiolactone

Article abstract of DOI:10.1021/jp108885s

The crystal structure of the six-member heterocyclic δ- valerothiolactone (1-thiocycloalkan-2-one) compound has been determined by X-ray diffraction at low temperature, revealing that its skeleton adopts a half-chair conformation. The conformation around the thioester group is almost planar with an anti orientation of the C=O double bond with respect the S-C single bond [C(2)-S(1)-C(6)-O(1) = 176.26(8)°]. The skeletal parameters, especially valence angles [∠-C5-C6-S = 121.19(6)°, ∠-O = C6-C5 = 122.25(8)°, ∠-C6-S-C2 = 106.80(4)°], differ from those typically found in acyclic thioester compounds, symptomatic of the presence of strain effects. The conventional ring strain energy was determined to be 7.5 kcal/mol at the MP2/6-311++G(d,p) level of calculation within the hyperhomodesmotic model approximation. Moreover, the valence electronic structure was investigated by HeI photoelectron spectroscopy assisted by quantum chemical calculations at the OVGF/6-311++G(d,p) level of theory. The first three bands at 9.35, 9.50, and 11.53 eV denote ionizations related with the n_S, n_O, and π_{C =O} orbitals, respectively, demonstrating the importance of the -SC(O)- group in the outermost electronic properties.

Full text of DOI:10.1021/jp108885s

12540
J. Phys. Chem. A 2010, 114, 12540–12547
Molecular and Electronic Structure of δ-Valerothiolactone
Nahir Y. Dugarte,† Mauricio F. Erben,*,† Roland Boese,‡ Mao-Fa Ge,§ Li Yao,§ and
Carlos O. Della Ve´dova*,†,|
CEQUINOR (UNLP-CONICET, CCT La Plata), Departamento de Qu´ımica, Facultad de Ciencias Exactas,
UniVersidad Nacional de La Plata, C.C. 962 (1900), La Plata, Repu´blica Argentina, LaSeISiC
(CIC-UNLP-CONICET) Departamento de Qu´ımica, Facultad de Ciencias Exactas, UniVersidad Nacional de La
Plata, Camino Centenario y 508, Gonnet, Repu´blica Argentina, Institut fu¨r Anorganische Chemie, UniVersita¨t
Duisburg-Essen, UniVersita¨tsstr. 5-7, D-45117 Essen, Germany, and State Key Laboratory for Structural
Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, China
ReceiVed: September 17, 2010; ReVised Manuscript ReceiVed: October 12, 2010
The crystal structure of the six-member heterocyclic δ-valerothiolactone (1-thiocycloalkan-2-one) compound
has been determined by X-ray diffraction at low temperature, revealing that its skeleton adopts a half-chair
conformation. The conformation around the thioester group is almost planar with an anti orientation of the
CdO double bond with respect the S-C single bond [C(2)-S(1)-C(6)-O(1) ) 176.26(8)°]. The skeletal
parameters, especially valence angles [∠C5-C6-S ) 121.19(6)°, ∠OdC6-C5 ) 122.25(8)°, ∠C6-S-C2
) 106.80(4)°], differ from those typically found in acyclic thioester compounds, symptomatic of the presence
of strain effects. The conventional ring strain energy was determined to be 7.5 kcal/mol at the MP2/6-
311++G(d,p) level of calculation within the hyperhomodesmotic model approximation. Moreover, the valence
electronic structure was investigated by HeI photoelectron spectroscopy assisted by quantum chemical
calculations at the OVGF/6-311++G(d,p) level of theory. The first three bands at 9.35, 9.50, and 11.53 eV
denote ionizations related with the nS, nO, and πCdO orbitals, respectively, demonstrating the importance of
the -SC(O)- group in the outermost electronic properties.
1. Introduction
prisingly, γ-butyrothiolactone does not react under similar
conditions. Therefore, the reactivity of the thiolactones toward
polymerization is dependent on the ring size in a manner that
is analogous to the behavior of lactones. In effect, it is well-
known that lactones, except γ-butyrolactone, are fairly reactive
and readily convert to their linear counterparts in the presence
of catalysts or initiators.15 The aliphatic polyesters prepared by
ring-opening polymerization of lactones are versatile polymers
with good mechanical properties that make them helpful for a
wide range of applications.16
Cyclic compounds containing one or more sulfur atoms are
widely used in synthetic, pharmaceutical, and agrochemical
products1 and also more recently have been discovered as
compounds with interesting physical properties.2,3 Thiolactones
are very interesting molecules in biological terms, one of the
most significant examples being the five-member cyclic ho-
mocysteine thiolactone molecule (DL-2-amino-4-mercaptobutyric
acid 1,4-thiolactone).4-6 Moreover, it is well-known that sulfur-
containing compounds play an important role in flavor chem-
istry,7 and the sensory characteristic of alkyl-substituted thi-
olactones has been evaluated.8 In recent years considerable
interest has been devoted to this class of compounds and the
synthesis of thiolactones has been reviewed by Paryzek and
Skiera.9 The five- and six-member γ-butyrothiolactone10 and
δ-valerothiolactone11 species, respectively, have been known
for several years and a number of the more stable alkyl-
substituted compounds derived from both types had already been
studied since almost 100 years ago.12 In particular, polymeri-
zation reactions of thiolactones have attracted much attention,
and the polymerization of four-member species have been
reported earlier.13 Overberger and Weise polymerized δ-vale-
rothiolactone and ε-caprothiolactone by a base-catalyzed ring-
opening reactions to linear, crystalline polythiolesters.14 Sur-
Our research group has quite recently started studies concern-
ing the structural, spectroscopic, and physicochemical properties
of simple thiolactones. The nonsubstituted four- and five-
member species (ꢀ-propiothiolactone17 and γ-butyrothiolac-
tone,18 respectively) have been studied by using a combined
experimental and quantum chemical approach. Thus, when these
molecules are isolated in a low-temperature inert Ar matrix,
the UV-visible (200 e λ e 800 nm) induced photochemistry
results in the formation of the corresponding ketene species,
methyl- and ethylketene, respectively. The valence electronic
structure has been also investigated by He(I) photoelectron
spectroscopy, and the first three bands for both molecules were
assigned to ionizations involving the nS, nO, and πCdO orbitals.
Moreover, the X-ray molecular structure has been elucidated
for ꢀ-propiothiolactone,17 and strain energies of 16.4 and 3.8
kcal/mol have been determined at the G2MP2 level for the four-
and five-member species, respectively.
* To whom correspondence should be addressed: Tel/Fax: +54-221-
425-9485. E-mail: erben@quimica.unlp.edu.ar (M.F.E), carlosdv@quimica.
unlp.edu.ar (C.O.D.V).
† CEQUINOR (UNLP-CONICET, CCT La Plata), Universidad Nacional
de La Plata.
In this study, we present an investigation of the structure,
electronic, and vibrational properties of the subsequent member
of this chemical family, the cyclic six-member δ-valerothiolac-
tone molecule. A comprehensive structural study has been
‡ Universita¨t Duisburg-Essen.
§ Chinese Academy of Sciences.
| LaSeISiC (CIC-UNLP-CONICET), Universidad Nacional de La Plata.
10.1021/jp108885s  2010 American Chemical Society
Published on Web 11/04/2010

Structure of δ-Valerothiolactone
J. Phys. Chem. A, Vol. 114, No. 47, 2010 12541
accomplished by measuring the low-temperature X-ray diffrac-
tion pattern using a laser-induced in situ crystallization method,
together with quantum chemical calculations at the B3LYP and
MP2 theories employing the 6-311++G(d,p) and aug-cc-pVTZ
basis sets. Vibrational spectra have been recorded for the liquid,
whereas the gas-phase photoelectron spectrum (PES) has been
obtained and interpreted within the orbital valence Green’s
functional (OVGF) theory. Finally, the strain energy was
determined within the s-homodesmotic model at the MP2/6-
311++G(p,d) level of approximation.
2. Experimental Section
2.1. Synthesis. δ-Valerothiolactone was prepared by using
benzyltriethylammonium tetrathiomolybdate as a sulfur-transfer
reagent, together with 5-bromovaleryl chloride, as was previ-
ously reported by Bhar et al.19 The final purity in the liquid
phase was carefully checked by reference to its IR, CG-MS,
Figure 1. Molecular model (with atom numbering) for the single-
crystal structure of δ-valerothiolactone.
1
and H and 13C NMR spectra.19
has been calculated by applying the s-homodesmotic approach
introduced by Zhao and Gimarc.28 To obtain the energies for
all acyclic systems, optimum equilibrium geometries were
computed for the singlet ground states of each and every one
of the pertinent molecular systems using B3LYP and MP2
methods with the 6-311++G(d,p) basis set. Several conforma-
tions were computed in order to ensure that the lowest energy
conformation was obtained for every species. In all cases,
electronic energies plus the zero-point energy were used to
compute the strain energies.
2.2. Instrumentation. IR absorption spectra in the liquid
state were recorded with a resolution of 1 cm-1 in the range of
4000-400 cm-1 using a Bruker model EQUINOX 55 equipped
with DLaTGS detector with a KBr window.
The PE spectrum was recorded on a double-chamber UPS-II
machine, which was designed specifically to detect transient
species, as described elsewhere,20-22 at a resolution of about
30 meV, indicated by the standard Ar+(2P3/2) photoelectron band.
Experimental vertical ionization energies were calibrated by
simultaneous addition of a small amount of argon and methyl
iodide to the sample.
3. Results
An appropriate crystal of δ-valerothiolactone of 0.3 mm
diameter was obtained on the diffractometer at a temperature
of 188(2) K with a miniature zone-melting procedure using
focused infrared laser radiation.23 The diffraction intensities were
measured at low temperature on a Nicolet R3m/V four-circle
diffractometer, the intensities being collected with graphite-
monochromatized Mo KR radiation using the ω-scan technique.
The structure was solved by Patterson syntheses and refined by
full-matrix least-squares on F with the SHELXTL-Plus pro-
gram.24 Absorption correction details are given elsewhere. All
atoms were assigned anisotropic thermal parameters. Further
details as well as atomic coordinates and equivalent isotropic
displacement coefficients for heavy and hydrogen atoms and
anisotropic displacement parameters (Å2 × 103) for δ-vale-
rothiolactone are given in Tables S1-S4 (Supporting Informa-
tion). Crystal structure data have been deposited at the Cam-
bridge Crystallographic Data Centre (CCDC). [Enquiries for data
can be directed to Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge, U.K. CB2 1EZ; e-mail, deposit@ccdc.
cam.ac.uk; fax, +44 (0) 1223 336033. Any request to the CCDC
for this material should quote the full literature citation and the
reference number 776608.]
3.1. Crystal Structure. Early studies of nonsubstituted
monocyclic lactones with different ring sizes showed that they
all polymerized except γ-butyrolactone, which is just on the
verge of polymerizability. The lack of polymerizability of
γ-butyrolactone was attributed to the low strain of the ring,
which shows much less geometric distortion in the ester group
than δ-valerolactone.15 Another factor influencing this behavior
is the stability of the conformations adopted by the linear
counterparts.29 Thus, the intrinsic correlation between structure
and reactivity has led to a better understanding of the chemical
reactivity of these cyclic compounds. In this sense, a broader
knowledge of the molecular structure of simple thiolactones is
of major interest. However, because thiolactones of small ring
size are liquids at ambient temperature, the information about
their molecular structure is rather scarce. The development of
special crystallization techniques has overcome this problem,
making possible the extension of the studies to obtain solid-
state information. By using the in situ laser-assisted crystal-
lization technique developed by Essen,23 an appropriate single
crystal of δ-valerothiolactone was grown at 188 K. The
j
compound crystallizes in the triclinic system (P 1 spatial group),
2.3. Quantum Chemical Calculations. The calculations
were performed using the Gaussian 0325 program package. Full
geometry optimizations were done by applying ab initio (MP2)
and DFT (B3LYP) methods using the 6-311++G(d,p) and aug-
cc-pVTZ basis sets. The calculated vibrational properties
corresponded in all cases to potential energy minima for which
no imaginary frequencies were found. The vertical ionization
energies (Ev) were calculated according to Cederbaum’s outer
valence Green’s function (OVGF) method26,27 with the 6-
311++G(d,p) basis set, based on the B3LYP/6-311++G(d,p)-
optimized geometry. The Mu¨lliken population analysis was
applied to assign the atomic charges for both neutral and
radical-cationic forms. The strain energy of the title compound
with the following unit cell dimensions: a ) 6.6573(1) Å, b )
6.9608(1) Å, c ) 7.2829(1) Å, R ) 73.188(1)°, ꢀ ) 77.789(1)°,
γ ) 62.946(1)°, and Z ) 2 (for the full crystallographic data
and treatment information, see Table S1 in the Supporting
Information).
The X-ray structure of the title compound is shown in Figure
1, where it is clearly observed that the molecule crystallizes in
the half-chair conformation, with the thioester group adopting
a nearly planar arrangement. Table 1 includes the main
geometric parameters derived from the structure refinement, as
well as those obtained from quantum chemical calculations.
The molecular structure shows a number of features of
interest. First, clearly there are two conflicting geometric

12542 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Dugarte et al.
TABLE 1: Experimental and Calculated Geometric
Parameters for the δ-Valerothiolactone
compromise to minimize the total energy. The value of the
internal ring angles at the carbonyl carbon [∠C5-C6-S )
121.19(6)°] and the alkyl sulfur [∠C6-S-C2 ) 106.80(4)°]
is considerably opened, as deduced by comparison with the
corresponding value of the bond angles of the acyclic related
species CH3C(O)SCH3 with values of 113.8(9)° and 99.2(9)°,
respectively, as determined from a gas electron diffraction
study.31 This distortion leads to a substantial strain energy for
this conformation, as will be discussed below.
B3LYP
aug-cc-
pVTZ
MP2
parametera
X-rayb
6-311++G(d,p)
6-311++G(d,p)
r(C6-S)
1.7681(8)
1.8230(9)
1.2048(10)
1.5111(14)
1.5201(13)
1.5215(12)
1.5100(12)
106.80(4)
113.33(6)
1.804
1.851
1.206
1.523
1.531
1.532
1.521
106.0
114.5
112.0
112.5
117.5
122.2
117.4
120.3
1.798
1.845
1.205
1.520
1.527
1.528
1.517
106.3
114.6
112.0
112.5
117.5
122.2
117.4
120.3
1.785
1.825
1.216
1.521
1.526
1.527
1.519
105.4
114.3
111.0
111.5
116.8
121.6
117.7
120.6
r(C2-S)
r(O-C6)
r(C2-C3)
r(C3-C4)
r(C4-C5)
r(C5-C6)
∠C6-S-C2
∠C3-C2-S
In general, the result of the two methods used in our study
to compute both bond lengths and bond angles compares fairly
well with the values obtained from the X-ray analysis. Some
deviations are found for the bond lengths around the sulfur atom,
and even with the large basis sets used in this study (aug-cc-
pVTZ), the B3LYP method overestimates the S-C bond length
(by up to 0.02 Å for the S-C2 bond and 0.03 Å for the S-C6
bond). Even though these bond parameters are better described
by the MP2 method with the modest 6-311++G(d,p) basis set,
this level of approximation fails to reproduce the CdO double
bond length and the OdC-C bond angle parameters, which
are calculated to be 0.01 Å longer and 0.7° smaller than the
experimental values, respectively. It is known, however, that
in the comparison of crystal and gas-phase structures, systematic
differences due to vibrational effects and intermolecular interac-
tions (packing effects) in the solid phase have to be considered,
especially for terminal bond lengths.31b Thus, considering these
∠C2-C3-C4 111.18(8)
∠C3-C4-C5 111.83(8)
∠C6-C5-C4 117.26(7)
∠OdC6-C5
∠OdC6-S
∠C5-C6-S
122.25(8)
116.47(7)
121.19(6)
a See Figure 1 for atom numbering. b Uncertainties are σ values.
requirements in this ring system. The thioester moiety, i.e., the
-C(2)SC(O)C(5)- group tends to remain planar having the
experimental dihedral angle -7.16(10)°. On the other hand, a
six-member ring tends also to adopt a chairlike conformation,
in order to allow the ethane units to present staggered confor-
mations.30 As a result of an energetic balance, both molecular
bond lengths and angles are substantially distorted in a
Figure 2. Stereoscopic illustration of the crystal packing of δ-valerothiolactone at 188 K.

Structure of δ-Valerothiolactone
J. Phys. Chem. A, Vol. 114, No. 47, 2010 12543
respectively. Only a single medium intensity absorption at 858
cm-1 is observed in the infrared spectrum of liquid δ-valerothi-
olactone in this region, suggesting the presence of one confor-
mation. The carbonyl stretching region shows the presence of
a number of broad bands in both spectra. It may be originated
by a conformational equilibrium or by association processes.
However, clear evidence for the presence of a boat conformation
in liquid δ-valerothiolactone is absent. Further studies should
be necessary to elucidate the conformational behavior, mainly
by using temperature variable experiments.
Figure 3. Molecular models computed [MP2/6-311++G(d,p)] for the
two main forms of δ-valerothiolactone.
3.3. Strain Energy. The evaluation of ring strain energies
in small cyclic compounds remains a topic of major interest.
The precise estimation of the effect that ring strain exerts upon
the chemical reactivity and the ground-state energy can be
interesting information for a synthetic chemist.34,35 A detailed
understanding of factors affecting the ring-opening polymeri-
zation of cyclic esters is the starting point to develop polymer
properties relevant to planned applications.34 The increase of
the molecular strain energy as function of the diminution of
the ring size in cyclic molecules is well-known.36 The strain
energy of δ-valerothiolactone has been computed within the
s-homodesmotic approach28,37 and compared with the related
ꢀ-propiothiolactone17 and γ-butyrothiolactone.18 The formal
reactions needed for computation of the ring strain for the title
molecule within this model are given in eqs 1-3.
systematic differences and the experimental error limits, ex-
perimental and computed MP2/6-311++G(d,p) are very similar.
Moreover, considering that only the gas-phase structure can
be reproduced by the optimized molecular geometry, significant
packing effects should be absent in solid δ-valerothiolactone
as deduced by the comparison between experimental and
computed results of Table 1. The overall crystal packing as
viewed along the bc plane is shown in Figure 2. The crystal
packing is governed by weak C-H· · ·OdC interactions that
connect molecules into two-dimensional networks. Molecules
in the unit cell are arranged forming two parallel chains with
the oxygen atoms oriented toward opposite directions, separated
by CdO· · ·H-C2 nonbonded distance of 2.65 Å, whereas the
distance between molecules within a chain adopts a value of
2.56 Å. Both interchain and intrachain molecular interactions
are given by CdO · · · H-C2 contacts, and in both cases the
hydrogen atoms are attached to the carbon atom (C2) bonded
to sulfur. This observance is in agreement with the crystal
structures of several polythiolactones reported recently.32 It was
found that both tubular and layered assemblies can be formed,
depending on the conformation adopted by the molecules.
3.2. Conformational Analysis. At this point, the comparison
of our results with those reported for the δ-valerolactone
molecule becomes interesting. For this species, the presence of
two conformers in the gas phase at room temperature, mainly
the half-chair and boat forms, was determined by microwave
spectroscopy.33 Both conformers are separated in energy by
approximately 0.6 kcal/mol and only the more stable half-chair
conformer can be observed in the crystal.32 Allinger and co-
workers33b also reported the presence of both conformations in
liquid δ-valerolactone by using Raman spectroscopy. In effect,
the ring-breathing vibrations appear as prominent signals at 748
and 798 cm-1 for the half-chair and boat conformations,
respectively.
For the title species, computed quantum chemical calculations
suggest that three structures are minima in the potential energy
hypersurface. In agreement with our results of the X-ray crystal
structure, the most stable computed conformation corresponds
to the half-chair form, with two energetically equivalent
enantiomers. The third conformation, which is 1.41 kcal/mol
(∆E0, B3LYP/aug-cc-pVTZ) higher in energy, corresponds to
the boat form. The calculated molecular structures are shown
in Figure 3. The computed (B3LYP/aug-cc-pVTZ) Gibbs free
energy difference between the half-chair and boat forms is ∆G0
) 1.30 kcal/mol, suggesting that a contribution of roughly 10%
for the boat form could be expected at room temperature. Even
though the theoretical energy difference for the title species is
higher than that of the oxygen analog, we carefully analyzed
the vibrational data, including the infrared and Raman spectra
(see Figure S1 in the Supporting Information) assisted by
frequency quantum chemical calculations for both forms. The
ring-breathing vibrations are computed (B3LYP/aug-cc-pVTZ)
at 853 and 840 cm-1 for the half-chair and boat forms,
The conventional strain energies determined with the B3LYP
and MP2 methods including zero-point corrections are given
in the Supporting Information (Table S5). As has been recently
reported by Ringer and Magers37 for cyclobutane, homodesmic
and hyperhomodesmic models result in similar strain energy
values, whereas the isodesmic reaction scheme yields strain
values that are definitively too low. These authors also pointed
out that the B3LYP method underestimates the strain energy.
All of these comments apply for our calculations on the strain
energy of δ-valerothiolactone.
The strain energy value calculated at the MP2/6-311++G(d,p)
level of approximation is 7.54 kcal/mol, a value much higher
than that found for cyclohexane38 (0.6 kcal/mol)sthe “reference”
molecule for a six-member cyclic speciessand also for cyclo-
hexanone34 (3.7 kcal/mol). However, the strain energy is lower
than that computed for the oxygen analogue δ-valerolactone,34
which amounts 10.2 kcal/mol. This behavior agrees with the
fact that the presence of a carbonyl group has little impact upon
the strain energy, but the replacement of a methylene group by
an oxygen atom increases the strain of the ring. Thus, the
tendency for the strain energy of six-member species related to

12544 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Dugarte et al.
TABLE 2: Comparison between the Strain Energy (kcal/
mol) of the Thiolactone with Their Respective Lactone
Species
thiolactones, the infrared and Raman spectra of δ-valerothi-
olactone have been measured in liquid phase (Figure S1,
Supporting Information), and the results obtained from the
theoretical calculations were used to assign these vibrational
spectra. The most intense band in the IR spectrum at 1664 cm-1
corresponds to the ν(CdO) and the signal at 601 cm-1 (591
cm-1 in Raman) can be assigned with confidence to the
ν(CH2-S)stretchingmodeofδ-valerothiolactone.Theν(S-CdO)
is absent in the infrared spectrum and observed as a medium
intensity signal in the Raman spectrum at 651 cm-1. This last
observation is in agreement with the quantum chemical calcula-
tions, which compute a very low absorption intensity at 655
cm-1 (B3LYP/aug-cc-pVTZ) for this vibrational mode. Vibra-
tional data for the ꢀ-propio-17,39 and γ-butyrothiolactones18 have
already been reported. Relevant vibrational data for these
thiolactone species are collected in Table 4. Contrary to the
behavior observed in lactones,35 the frequency values for the
ν(CH2-S) and ν(S-CdO) normal modes of vibration slightly
decrease when the size of the thiolactone ring increases from
ꢀ-propio- to δ-valerothiolactone. This difference could be
attributed to the fact that ring strain energies (and the geometry
distortions) are much higher for the oxygen analog, and thus,
the influence exerted by the ring strain is a dominant factor.
For thiolactones, other factors also depending on the size of
the carbonated chain [for instance, the inductive effect exerted
by the -(CH2)n- group] seems to influence the force constants
of the -SC(O)- moiety.
ꢀ-propio
(ring size ) 4)
γ-butyro
(ring size ) 5)
δ-valero
(ring size ) 6)
lactonea
thiolactone
22.7
7.8
10.2
7.5c
16.4b 17
3.8b 18
a Calculated values from ref 34 at the CBS-Q level of
approximation. b G2MP2 values. c Calculated with MP2/
6-311++G** (this work).
title compound is δ-valerolactone > δ-valerothiolactone >
cyclohexanone > cyclohexane.
The ring strain energy value of the cyclic thioester molecule
studied in the present work is compared with their respective
cyclic oxoester (lactones) in Table 2, where a good correlation
in the trend for both families is observed. The lowest strain
energy for both lactone and thiolactone families corresponds
to the five-member ring species, the corresponding value for
the six-member ring being notably higher. The increase of the
number of atoms in the ring allows the carbonyl group and the
sulfur atom to adopt bond angle values closer to the geometry
expected for the acyclic nonstrained species. However, the
conformation requirement imposed by the ring also affects the
neighboring methylene groups. These aspects have been clearly
discussed in the described molecular structure of the title species.
Experimental and calculated [B3LYP/6-311++G(d,p)] bond
angles for lactone and thiolactone species are given in Table 3.
With exception of the bond angles around the heteroatom
∠C-X-C(O), similar values are found ((3.5°) when the
parameters of both families are compared. The values are higher
than 109.5° for this six-member ring, amounting to 117.5° and
115.7° for the ∠C(O)-C-C of δ-valerothiolactone and δ-vale-
rolactone, respectively.
3.4. He I Photoelectron Spectra. The PE spectrum of
δ-valerothiolactone is presented in Figure 4. The experimentally
observed ionization energies (IP in eV), calculated vertical
ionization energies at OVGF/6-311++G(d,p) (Ev in eV),
molecular orbitals, and molecular characters are summarized
in Table 5.
Two bands appear overlapped at 9.35 and 9.50 eV; their
narrow and sharp contours are characteristic of ionizations from
essentially nonbonding orbitals, which suggest that these bands
are linked primarily with the ionization process from both sulfur
and oxygen lone-pair electrons (nS, nO) of the -SC(O)- group,
respectively. This assignment is in agreement with the results
of ab initio calculations, with respect to the ordering of the
orbitals, as shown in Table 5 and Figure 5. The same orbital
In lactones, the effect of the distortion in the molecular
structure was noted in the vibrational spectra reported by
Saiyasombat and co-workers.35 The normal modes related with
the lactone group are sensitive toward the strain energy, and an
increment in the frequency values of the ν(CH2-O) and
ν(O-CdO) stretching modes is noted when the ring size
increases. To elucidate whether a similar effect is valid for
TABLE 3: Comparison of the Valence Bond Angles between Lactones (X ) O) and Thiolactones (X ) S)
bond angle (deg)
C-X-(CdO)
X(CdO)CR
(CdO)CR-Cꢀ
CR-Cꢀ-Cγ
calc exp
Cꢀ-Cγ-Cδ
calc exp
C-C-X
calca
expb
calc
exp
calc
exp
calc
exp
ꢀ-propiothiolactone
ꢀ-propiolactone
γ-butyrothiolactone
γ-butyrolactone
δ-valerothiolactone 106.0 106.80
δ-valerolactone 122.6
76.4
91.9
93.1
77.27(5)
94.9
94.2
110.0
109.1
94.55(7)
95.7
83.9
108.3
103.7
95.68(9)
92.9
90.1
106.0
105.5
92.42(7)
107.3
101.9
111.0
120.3 121.19(6) 117.5 >117.26(7) 112.5 111.83(8) 112.0 111.18(8) 114.5 113.33(6)
118.7 115.7 109.2 109.2 113.7
a Values computed at the B3LYP/6-311++G** level of approximation. b Experimental X-ray molecular structure for ꢀ-propiothiolactone17
and δ-valerothiolactone (this work).
TABLE 4: Selected Vibrational Data (in cm-1) for ꢀ-Propio-,17,39 γ-Butyro-,18 and δ-Valerothiolactones
ꢀ-propiothiolactone17,39 (ring size ) 4)
γ-butyrothiolactone (ring size ) 5)18
δ-valerothiolactone (ring size ) 6)a
IRb
Ramanc
calcd
IRc
Ramanc
calcd
IRd
Ramanc
calcd
ν(CdO)
ν(S-CH2)
ν(S-CdO)
1785.0
743.6
652.7
1759
739
662
1855
739
652
1704
685
628
1702
687
632
1793
681
617
1663
1663
651
591
1754
654
580
601
a This work. b Ar-matrix. c Liquid phase. d B3LYP/6-311++G** level of approximation.

Structure of δ-Valerothiolactone
J. Phys. Chem. A, Vol. 114, No. 47, 2010 12545
Figure 6. Correlation diagram of the ionization potentials of ꢀ-pro-
piothiolactone, γ-butyrothiolactone, and δ-valerothiolactone.
Figure 4. Photoelectron spectrum of δ-valerothiolactone.
TABLE 5: Experimental Vertical Ionization Energies (IP in
eV) and Computed Ionization Energies (Ev in eV) at the
OVGF/6-311++G(d,p) Level of Approximation and
Molecular Orbital Characters for δ-Valerothiolactone
and the atomic charges are mostly delocalized all over the
molecule, with an appreciable fraction localized at the -C(O)S-
group, as is listed in Table S6 in the Supporting Information.
The cyclic planar (C1) molecular geometry is conserved after
the ionization. The C6-S bond length is 0.29 Å longer,
while the CdO double bond becomes 0.04 Å shorter in the
cationic state.
IP (eV)
Ev (eV)a
MO
character
9.35
9.50
9.01
9.57
(31)
(30)
nS
nO
11.53
11.96
11.29
12.23
12.31
12.94
13.19
14.31
14.73
15.27
15.46
(29)
(28, 27)
πCO
σC4-Hs
There is a relatively large gap between the second and the
third band, the last having an ionization energy of 11.53 eV.
The calculations predict that this band is associated with an
ionization of electron mainly localized on the πCO orbital, as
depicted in Figure 5. A group of overlapping bands appears
between 12 and 16 eV, which can be assigned to ionizations
from several σ-CH2 orbitals, as predicted by the calculations.
3.5. Correlation Diagrams. The first three IP’s of the
δ-valerothiolactone molecule are correlated with the corre-
sponding values determined for ꢀ-propiothiolactone17 and
γ-butirothiolactone,18,40 in the diagram shown in Figure 6. The
prominent feature is the diminution of the ionization energy of
the three outermost orbitals when the ring size increases.
Resonance or mesomeric effects in the -C(O)S- group promote
a local planar geometry and are dominated by the donor-acceptor
12.79
13.13
14.14
14.89
15.16
(26,25)
(24,23)
(22,21)
σC3-Hs
σC5-Hs
σC2-Hs
a Molecular geometry calculated at the B3LYP/6-311++G(d,p)
level of approximation.
trend has been reported for the four- and five-member thiolac-
tone species17,18,40 and also for related acyclic molecules, for
example, thioacetic acid,41 S-alkyl thioacetates,42 and XC(O)SCl
(X ) F43 and Cl44) species. For the first band (nS), the value
observed for the vertical ionization (9.35 eV) is in reasonable
agreement with the theoretically predicted value of 9.01 eV
[ROVGF/6-311++G(d,p)]. Further calculations [UB3LYP/6-
311++G(d,p)] have been performed in order to analyze the
nature of the low-lying radical-cation formed in the first
ionization process. The adiabatic ionization energy is 8.92 eV
interactions from the out-of-plane lone pair nS orbitals to the
45
vacant π*CdO
.
The nonplanarity of the five- and six-member
thiolactones ring would prevent this interaction. Thus, in order
to consider the most important intramolecular interactions
present in these species, the donor-acceptor interaction energies
for ꢀ-propio-, γ-butyro-, and δ-valerothiolactone species have
Figure 5. Drawings of three HOMOs (highest occupied molecular orbitals) of δ-valerothiolactone.

12546 J. Phys. Chem. A, Vol. 114, No. 47, 2010
Dugarte et al.
been evaluated by using the NBO population analysis. Two
energetically relevant interactions around the -SC(O)- group
are specially analyzed:45,46 (a) “mesomeric interaction” or the
charge transfer from the lone-electron pair with π symmetry
(n′′S) to the π*CdO formally vacant orbital, which serve to
commensurate the electronic delocalization within the -SC(O)-
fragment, and (b) “anomeric interaction” or the charge transfer
from the in-plane p-type lone electron pair of the sulfur atom
(n′S) to the σ*C-C antibonding orbital.45 At the B3LYP/6-
311++G(d,p) level, the contributions of these interactions
amount to 36.2, 36.4, and 34.0 kcal/mol for ꢀ-propio-, γ-butyro-
and δ-valerothiolactones, respectively. These results suggest that
the electronic interaction is not much affected by the nonpla-
narity of the thiolactone ring. The local planar symmetry around
the carbonylic sp2 carbon atom leads to an efficient electronic
conjugation. These results are in agreement with the early
suggestion of Chin et al. These authors pointed out that the
electronic properties of thiolactones are determined primarily
tively. The FTIR and FT-Raman spectra are shown in Figure
S1. This material is available free of charge via the Internet at
http://pubs.acs.org.
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A
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The X-ray molecular structure of δ-valerothiolactone reveals
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Acknowledgment. M.F.E. and C.O.D.V. are members of the
Carrera del Investigador del CONICET, Repu´blica Argentina.
The Argentinean authors thank the Consejo Nacional de
Investigaciones Cient´ıficas y Te´cnicas (CONICET), the Agencia
Nacional de Promocio´n Cient´ıfica y Tecnolo´gica (ANPCYT),
and the Comisio´n de Investigaciones Cient´ıficas de la Provincia
de Buenos Aires (CIC), Repu´blica Argentina. They are indebted
to the Facultad de Ciencias Exactas, Universidad Nacional de
La Plata for financial support. Financial support by the Volk-
swagen-Stiftung and the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. C.O.D.V. and N.Y.D. especially
acknowledge the DAAD, which generously sponsors the DAAD
Regional Program of Chemistry for the Repu´blica Argentina
supporting Latin-American students to study for their Ph.D. in
La Plata.
Supporting Information Available: Crystallographic data
and listing of atomic coordinates and equivalent isotropic
displacement coefficients and anisotropic displacement param-
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