CN Stretching Wavenumber as a Vibrational Si - American Chemical

Article pubs.acs.org/JPCB

Time-Resolved IR Spectroscopy of 1,3-Dicyanophenylcyclopentane1,3-diyl Diradicals: CN Stretching Wavenumber as a Vibrational Signature of Radical Character Akihiro Maeda,† Takahide Oshita,† Manabu Abe,*,†,‡ and Taka-aki Ishibashi*,§ †

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan ‡ Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan § Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571, Japan S Supporting Information *

ABSTRACT: CN stretch bands of singlet and triplet cyclopentane-1,3-diyl diradicals (1,3-di(4-cyanophenyl)-2,2-dimethoxyoctahydropentalene-1,3-diyl and 1,3-di(4-cyanophenyl)-2,2-dimethyloctahydropentalene-1,3-diyl) were observed by time-resolved IR spectroscopy. CN stretching wavenumbers of the singlet and triplet diradicals were downshifted by 10 and 19 cm−1 from those of their corresponding ring-closed compounds, which are in closed-shell electronic states, respectively. The observed downshifts are attributed to the bond-order decrease in the CN bonds due to the contribution from a resonance structure that has cumulative double bonds (CCN•) at the para-positions of the radical carbons. This resonance structure is only possible when the molecules have unpaired electrons; thus, the wavenumber downshift can be regarded as an experimental manifestation of the radical character of the diradicals. The observed CN stretching wavenumbers indicate that the radical character of the singlet diradical is less significant than that of the triplet. The smaller radical character is ascribed to the contribution from the zwitterionic and π-single bonding resonance structures in the singlet diradical. Unrestricted DFT calculations at the B3LYP level of theory with the 6-31G(d) basis set reproduced the small/large relationship between the wavenumber downshifts of the singlet and triplet diradicals; however, the shift of the singlet diradical was overestimated.

1. INTRODUCTION

established to control their ground-state spin-multiplicity and stability.3,4,6−22 Electronic absorption spectroscopy and quantum chemical calculations have thus far primarily been utilized in studies of singlet cyclopentane-1,3-diyl diradicals. They exhibit characteristic absorption at approximately 600 nm, which is convenient for the identification of singlet diradicals of this type. Furthermore, solvation11 and substituent effects13 on the stability of singlet cyclopentane-1,3-diyl diradicals have been discussed on the basis of resonance structures such as π-singlebonding (Figure 1, I) and zwitterionic structures (Figure 1, III). However, experimental evidence to support the discussion is rather scant. Therefore, we have conducted IR study to elucidate the structural or bonding characteristics of 1,3diphenylcyclopentane-1,3-diyl diradicals. The target diradicals in this study are 1,3-di(4-cyanophenyl)2,2-dimethoxyoctahydropentalene-1,3-diyl (DR1)13 and 1,3di(4-cyanophenyl)-2,2-dimethyloctahydropentalene-1,3-diyl

Diradicals are molecules that have two unpaired electrons occupying two nearly degenerate molecular orbitals (MOs).1−4 When the MOs are spatially localized, the diradicals are said to be localized. Diradicals are, in general, reactive and short-lived because of the unpaired electrons. Singlet and triplet spin multiplicities are possible for diradicals, and localized singlet diradicals have been attracting significant attention because they serve as models for the intermediate of homolysis.3,4 Cyclopentane-1,3-diyls with various substituents are one of the most extensively studied localized diradicals. In 1975, Buchwalter and Closs observed the EPR spectrum of cyclopentane-1,3-diyl, confirming that it is a triplet localized diradical.5 Approximately 20 years later, guided by a theoretical prediction,6 Adam et al. reported the transient electronic absorption of 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyl with a lifetime of 80 ns in n-pentane at room temperature.7 This was the first experimental observation of a singlet 1,3diradical. Since then, much effort has been focused on generating more long-lived singlet cyclopentane-1,3-diyls. Several useful chemical modification methods have been © 2014 American Chemical Society

Received: January 19, 2014 Revised: March 11, 2014 Published: March 26, 2014 3991

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

laser (10 ns duration, Spectron, SL401). The induced change in the MCT output due to photoexcitation of the sample was amplified with AC-coupled amplifiers and was then accumulated in a digital sampling oscilloscope (Lecroy, LT342L) until approximately 10 μs after the photoexcitation. The temporal profiles at different wavenumbers were reconstructed as TRIR spectra. The spectral resolution was set to 8 cm−1, and the spectral data were sampled at intervals of 2 cm−1. The time resolution of the detection system was limited to 35 ns by the MCT detector. Diradicals DR1 and DR2 were generated by photoexciting dichloromethane (Kanto Chemical Co., >99.5% GC grade, 0.01 mol dm−3) solutions of corresponding azo-compounds, AZ1 and AZ2, respectively. AZ1 and AZ2 were synthesized by previously reported methods.13,23 For measurements, the solutions were saturated with nitrogen gas to remove molecular oxygen unless otherwise stated. An IR flowing cell (100 μm optical path length) made of two CaF2 windows and a thin lead spacer was used for sampling. Vibrational wavenumbers of the diradicals, azo-species, and ring-closed species were calculated using a DFT method. All calculations were carried out by Gaussian 09.28 The B3LYP exchange-correlation functional and 6-31G(d) basis set functions were used. Restricted wave functions (RB3LYP) were used for the azoalkane species and ring-closed species, while unrestricted wave functions combined with the brokensymmetry (BS) technique22,29−31 (BS-UB3LYP) were used for the calculation of the singlet and triplet diradicals. The BS technique is often used to reproduce the electronic states of molecules with open-shell singlet states, such as singlet diradicals.

Figure 1. Resonance structures of singlet 2,2-dialkoxycyclopentane 1,3-diyl diradicals.

(DR2),23 which have the cyclopentane-1,3-diyl framework with p-cyanophenyl groups on C1 and C3 (Figure 2). Depending on

Figure 2. Molecular structures of singlet diradical DR1 and triplet diradical DR2.

the substituent groups on C2, the electronic ground state spin multiplicity of the former is singlet, while that of the latter is triplet. Both diradicals (DR1 and DR2) are generated via photoinduced denitrogenation from the corresponding azoalkanes (AZ1 and AZ2) and are then converted into ring-closed compounds (CP1 and CP2, respectively; Figure 3). The

3. RESULTS AND DISCUSSION 3.1. Time-Resolved IR Absorption Spectra. The vibrational spectra of the singlet and triplet diradicals (DR1, DR2) in the CN stretching region were observed in dichloromethane. DR1 and DR2 were generated from the corresponding azoalkanes, AZ1 and AZ2, respectively. The observed TRIR difference spectra of the singlet diradical (DR1) are shown in Figure 4b with a stationary spectrum of AZ1 in dichloromethane (Figure 4a). The TRIR spectra of the triplet diradical (DR2) and a stationary spectrum of AZ2 are shown in Figures 4d and 4c, respectively. The ordinate of the TRIR spectra is the photoinduced absorbance change; positive and negative signals correspond to the increase and decrease upon photoexcitation, respectively. The series of TRIR spectra of DR1 and those of DR2 correspond to the generation and decay processes of the diradicals. Singlet Diradical DR1. First, we discuss the TRIR spectra of the singlet diradical (DR1). Upon photoexcitation by a 266 nm laser pulse, the band at 2234 cm−1 of the azoalkanes (AZ1) decreased instantaneously (Figure 4b). At the same time a new band appeared at 2219 cm−1, which was assignable to the singlet diradical. The peak position of the upward band shifted gradually with time toward larger wavenumbers and eventually reached 2225 cm−1. This band shifting corresponds to the unimolecular reaction from the diradical to the ring-closed compound CP1, and the 2225 cm−1 band is ascribed to CP1 (Figure 3). We identified only one CN stretch band for each species (AZ1, DR1, CP1) though they have two CN stretch modes (symmetric and antisymmetric). There are two conceivable and related reasons for the discrepancy between the numbers of the

Figure 3. Generating and decaying processes of the target diradicals (DR1 and DR2).

electronic structure of the CN bonds of the diradicals are expected to be influenced by radical electrons because the CN groups are located at the para-position of the radical centers C1 and C3. We found that the CN stretching wavenumbers of diradicals DR1 and DR2 were quite different from each other and much smaller than those of ordinary closed-shell molecules (AZ1, AZ2, CP1, and CP2). The observed large downshifts of the diradicals will be interpreted using their resonance structures. The large downshift will also be proposed as a measure of the radical character of the singlet and triplet diradicals.

2. EXPERIMENTAL SECTION Transient IR absorption spectra were recorded for the wavenumber region 2260−2180 cm−1 with a home-built time-resolved IR (TRIR) spectrometer.24−27 The TRIR spectrometer consisted of a continuous-wave ceramic IR light source, grating monochromator (focal length, 50 cm; grating, 100 grooves/mm; JASCO, CT50TF), and a photovoltaic MCT (HgCdTe) detector (Kolmar; KMPV11-1-J1). A sample solution was photoexcited by 266 nm pulses (2.5 mJ/cm2, 2 Hz) from the fourth harmonic light of a Q-switched Nd:YAG 3992

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

R(τ ) = π −1/2σ −1e−τ 2

AI(ν)̃ = AIe−(ν ̃− νĨ )

2

/σ 2

/ wI 2

,

,

2

AR (ν)̃ = AR e−(ν ̃− νR̃ ) 2

AP(ν)̃ = APe−(ν ̃− νP̃ )

/ wR 2

,

/ wP 2

where AR(ν̃), AI(ν̃), and AP(ν̃) are the CN stretch band shape functions of the reactant, intermediate, and product, respectively. R(τ) is a temporal response function (σ = 2.15 × 10−8 s, i.e., fwhm = 35.8 ns) of the spectrometer, and θ(t) is the step function. The CN stretch bands were assumed to have Gaussian band shapes with amplitudes that followed the firstorder reaction kinetics shown in Figure 3. We also presumed that the photoinduced denitrogenation process was much faster (k1 ≫ 1011 s −1) than the temporal response of the spectrometer; thus, it can be seen as an instantaneous process.7 The fitting result for the dimethoxy species (AZ1, DR1, CP1) is shown as dashed lines in Figure 4b. Note that the model function well reproduced the series of observed TRIR spectra. The extracted CN stretch bands (AR(ν̃), AI(ν̃), and AP(ν̃)) are shown in Figure 5a, and the lifetime of intermediate

Figure 4. IR spectra of (a) AZ1 and (c) AZ2 before UV-irradiation and time-resolved difference IR spectra of (b) AZ1 and (d) AZ2 in dichloromethane (0.01 mol dm−3) after the laser flash photolysis (266 nm, 2.5 mJ/cm2, 2 Hz) at 20 °C. The TRIR spectra are difference absorbance between before and after the photoexcitation; the positive signal corresponds to the increase in absorption, while the negative one corresponds to the decrease. The sample solutions were saturated with nitrogen gas.

observed stretch bands and the stretch modes. First, the antisymmetric mode is much stronger in intensity than the symmetric mode. Second, the wavenumber difference between the symmetric and antisymmetric bands is smaller when compared to the observed bandwidth. We consider three possible cases for this observed discrepancy: (1) only the first reason is true, (2) only the second reason is true, or (3) both the first and second reasons are true. The DFT calculation showed very small wavenumber differences for AZ1, DR1, and CP1: 0.2, 0.3, and 1.7 cm−1, respectively. They were sufficiently smaller than the wavenumber resolution (8 cm−1) of the spectrometer. The calculated IR intensity ratios between the two modes (antisymmetric: symmetric) of AZ1, DR1, and CP1 were 5:1, 13:1, and 4:3, respectively. The difference in the calculated intensity ratios may mainly due to the different angles between the two CN bonds among the species. The calculated angles of the azo compound and the diradical were large (140°−150°), while that of the ring-closed products was small (≈90°). The angles are consistent with the fact that the ratios of the former species were large and those of the latter species was small. (See Figure S5 in the Supporting Information for the calculated angles.) On the basis of the calculated ratio, the observed CN bands of AZ1 and DR1 were assigned to the antisymmetric CN stretch mode, while that of CP1 was contributed by the antisymmetric and symmetric modes. Detailed results of the DFT calculation are summarized in Table S1 of the Supporting Information. To deduce the lifetime and peak position of the vibrational band of the singlet diradical DR1, we conducted a global fitting analysis of the TRIR spectra (Figure 4b). The model functions used are as follows:

Figure 5. Band shapes of AR, AI, and AP obtained from fitting analysis of time-resolved difference IR spectra of (a) AZ1 and (b) AZ2. The number above each band indicates the wavenumber of each band, and the number in each band indicates the band area that is proportional to the molar extinction coefficient of the species.

k2−1 was determined to be 1.0 μs. We also examined the effect of the oxygen gas on the lifetime by using a sample solution saturated with oxygen gas and confirmed that the lifetime was hardly affected by the oxygen gas, supporting that the observed transient species was the singlet diradical DR1 rather than a triplet species.13 The lifetime determined in dichloromethane by TRIR, 1.0 μs, was a little longer than that estimated in benzene by electronic absorption spectroscopy, 630 ns.13 The lifetime prolongation was most likely due to a solvent effect. A previous study11 of 2,2-diethoxy-1,3-diphenyloctahydropentalene-1,3diyl (DR3, Figure 6) revealed that the singlet diradical was stabilized in polar solvents, and the lifetime of DR3 in dichloromethane was 1.8 times longer than that in benzene. If the lifetime prolongation factor of DR1 is transferable from DR3, then the observed lifetime of 630 ns in benzene corresponds to that of 1.1 μs in dichloromethane, which is

t

ΔA(ν ̃, t ) =

∫−∞ R(τ)[−AR (ν)̃ + AI(ν)ẽ −k (t−τ) 2

+ AP(ν)(1 − e−k 2(t − τ))]θ(t − τ ) dτ ̃

Figure 6. Molecular structure of DR3. 3993

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

deduced by the fitting of the TRIR spectra. The most important observation is that the CN wavenumbers of the diradicals were much smaller than those of the azo and ring-closed compounds, suggesting that the CN bond character of the diradical species is significantly affected by the presence of the unpaired electrons. It should also be noted that the CN wavenumber difference between the dimethoxy-substituted diradical (DR1) and the dimethyl-substituted diradical (DR2) was also significant: the CN wavenumber of DR2 was smaller than that of DR1 by 8 cm−1. However, the CN wavenumbers of the azoalkanes (AZ1 and AZ2) and the ring-closed compounds (CP1 and CP2) were quite insensitive to whether the substituents were dimethoxy or dimethyl groups. This implies that the CN wavenumber downshift of DR2 (triplet) from DR1 (singlet) originates in the difference of the spin multiplicity. The observed downshift of the CN stretch band of the diradicals may be accounted for by a resonance structure of para-cyano-substituted benzyl radicals (Figure 7). The con-

quite similar to the obtained lifetime of DR1 in this study (1.0 μs). Triplet Diradical DR2. The generation and decay processes of the triplet diradical DR2 were observed in dichloromethane by TRIR spectroscopy. The observed TRIR spectra are shown in Figure 4d with the stationary spectrum of AZ2 in Figure 4c; AZ2 was converted into DR2 by the photoinduced denitrogenation process. Although the stationary spectrum of dimethyl-substituted azo-species (AZ2) is quite similar to that of the dimethoxy-substituted azo-species (AZ1), the spectra and decay behavior of the transient 2,2-dimethyl-1,3-diradical DR2 were different from those of the 2,2-dimethoxysubstituted 1,3-diradical DR1. The CN stretch band of the dimethyl-substituted transient DR2 appeared at 2210 cm−1, which is a much lower wavenumber position compared to the dimethoxy-substituted singlet diradical DR1. The band evidently decayed to the final product CP2 through a slower process than that of DR1. The TRIR spectra of DR2 (Figure 4d) were analyzed in exactly the same manner as those of DR1 (Figure 4b) to deduce the band shape and kinetic parameters. The fitted spectra are shown as dashed lines in Figure 4d and the obtained CN band shapes of AZ2, DR2, and CP2 in Figure 5b. The CN stretch band positions of AZ2, DR2, and CP2 were determined to be 2234, 2210, and 2229 cm−1, respectively, and the lifetime of DR2, k2−1, in dichloromethane to be 6.2 μs. On the basis of the calculated IR intensity ratios between the two modes (antisymmetric: symmetric) of AZ2, DR2, and CP2 (11:1, 20:1, and 4:3, respectively), the observed CN bands of AZ2 and DR2 were assigned to the antisymmetric CN stretch mode, while that of CP2 was contributed by the antisymmetric and symmetric modes. The lifetime (6.2 μs) of DR2 supports its assignment to the triplet diradical. First, it was similar to a previously reported value, 9.6 μs, which was observed in benzene saturated with nitrogen gas by electronic absorption spectroscopy.23 The difference between the lifetimes observed by IR and electronic spectroscopies may be attributed to a heavy atom effect of the solvent (i.e., the chlorine atoms in the dichloromethane), where the intersystem crossing would be accelerated by the spin− orbit coupling mechanism. The stabilization of the singlet state of DR2 in polar dichloromethane11 is another possible origin of the lifetime shortening of the triplet DR2. The converting process of the triplet DR2 to the corresponding ring-closed compound is assumed to proceed as follows: the triplet species is thermally activated to the singlet state of DR2, and then the ring-closing process occurs on the singlet potential surface. Stabilization may increase the thermal population of the singlet DR2 and hence accelerate the ring-closing process. Second, the lifetime of DR2 was found to be greatly shortened to 0.57 μs in dichloromethane saturated with molecular oxygen. The accelerated decay of DR2 is attributed to a reaction of the triplet DR2 with oxygen to form the corresponding peroxide product,23 which is consistent with the assignment of DR2 to the triplet species. 3.2. Observed CN Stretching Wavenumbers. Here, the CN stretching wavenumbers (Figure 5) of diradicals DR1 and DR2, azoalkanes AZ1 and AZ2, and ring-closed compounds CP1 and CP2 observed by TRIR are discussed. The order of decreasing wavenumbers is as follows: azoalkanes (AZ1, 2235; AZ2, 2234) > ring-closed derivatives (CP2, 2229; CP1, 2228) > singlet diradical (DR1, 2218) > triplet diradical (DR2, 2210). The numbers in parentheses represent wavenumbers in cm−1

Figure 7. Resonance structures for para-cyano-substituted benzyl radical derivatives.

tribution of the quinoid resonance structure with cumulative double bonds (CCN) weakens the CN bond strength and thus lowers the CN stretching wavenumber. The observed smaller wavenumber of the triplet diradical DR2 than that of the singlet diradical DR1 suggests that the radical character, e.g. the spin density at C1 and C3 carbons, in the singlet diradical is lower than that in the triplet diradical. The smaller contribution of the structure with a CCN• group in the singlet state is reasonable because the localized singlet diradicals possess other resonance structures such as I and III in Figure 1. The bonding interaction between C1 and C3 carbons in the π-single bonding structure (Figure 1, I) and the zwitterionic structure (Figure 1, III) are possible only for the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradical DR1. Thus, the CN stretching wavenumber is considered as an indicator of the extent of radical character. 3.3. Calculated CN Stretching Wavenumbers. Here, the observed CN stretching wavenumbers of the singlet and triplet diradicals are compared with the wavenumbers calculated by DFT methods. This comparison has two purposes: (1) to assess the accuracy of DFT calculations on the vibrational analysis in cyclopentane-1,3-diyl diradicals and (2) to rate the radical character of the singlet and triplet diradicals by comparing the downshift of the CN stretching wavenumbers of the diradicals with that of an aromatic mono radical species with a nitrile group whose structure is a substructure of the diradicals. The accuracy of vibrational analysis based on DFT calculation for cyclopentane-1,3-diyl diradicals is unknown because of a lack of experimental data. To the best of our knowledge, this paper reports the first observation of the vibrational spectra of cyclopentane-1,3-diyl diradicals. DFT calculations based on the B3LYP method with appropriate basis sets have been established to give reliable electronic energies and vibrational wavenumbers of the closed-shell electronic 3994

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

ground state of organic molecules.32 However, the accuracy of the B3LYP method may be limited for open-shell molecules especially singlet diradical species whose electronic structures are difficult to describe with a single-determinant wave function. For cyclopentene-1,3-diyl diradicals, unrestricted DFT calculations often predict the correct energy order between the lowest singlet and triplet states; however, the square of the electronic spin, ⟨S2⟩, of the singlet state is poor.14−17 Vibrational analyses were conducted for the diradicals (singlet DR1, triplet DR2), azoalkanes (AZ1, AZ2), and ringclosed compounds (CP1, CP2) using the B3LYP method with the 6-31G(d) basis set. Unrestricted wave functions were used for open-shell species (diradicals), and restricted wave functions were used for closed-shell species (azoalkanes and ring-closed species). The calculated energy orderings between the lowest singlet and lowest triplet states of the diradicals at the optimized geometries were consistent with the experiments.13,23 The singlet was 7.5 kJ/mol more stable than the triplet for dimethoxy-substituted diradical (DR1), while the triplet was 0.66 kJ/mol more stable than the singlet for dimethyl-substituted diradical (DR2).33 (Refer to Table S2 in the Supporting Information for further details.) The calculated CN stretching wavenumbers are compared with the observed wavenumbers in Table 1, which also includes the calculated and

number of the CN stretch mode of CP1 matched the observed wavenumber. The calculation reproduced the observed order of the wavenumbers of various species: azoalkanes (AZ1, AZ2) > ring-closed derivatives (CP1, CP2) > singlet diradical DR1 > triplet diradical DR2. However, quantitatively, there are two drawbacks of the DFT results. First, the calculated wavenumber shift of singlet DR1 from CP1 was overestimated as −17 cm−1 as compared to the observation (−10 cm−1), though that of triplet DR2 from CP2, −19 cm−1, reproduced the experimental value. This deviation may be due to spin contamination in the unrestricted wave function of the singlet diradical DR1. The calculated ⟨S2⟩ for the singlet DR1 was 0.95, which is close to the middle value between the ideal value for the singlet, 0, and that for the triplet, 2 (Table 2). On the other hand, ⟨S2⟩ for the Table 2. Wavenumber Shiftsa and ⟨S2⟩ of DR1 Calculated with Different Methods

intensityc

obs.

calc.d,e

obs.

calc.d

obs.

calc.

DR1 DR2 CP1 CP2 AZ1 AZ2 3 4

2218 2210 2228 2229 2235 2234

2211 2209 2228 2228 2229 2229 2208 2228

1.0 1.1 0.56 0.28 0.34 0.19

153 175 102 101 96 91 75 44

−10 −19

−17 −19

energy (au)

⟨S2⟩

spin multiplicity

BS-UB3LYP UB3LYP

−17 −18

−1187.6387 −1187.6357

0.95 2.06

singlet triplet

Wavenumber shifts were calculated from ring-closed species CP1. Observed wavenumber shift of DR1 was −10 cm−1.

triplet DR1 was calculated as 2.06, which is a reasonable value for the triplet state. The ⟨S2⟩ value of the singlet diradical suggests that the calculated electronic structure of the singlet DR1 has considerable contamination of the triplet state, which may bring the calculated CN stretching wavenumber of the singlet DR1 close to that of the triplet DR1. The failure of the calculated shift for the singlet diradical indicates that the unrestricted DFT wave function is insufficient in reproducing vibrational wavenumbers of singlet diradical species such as cyclopentane-1,3-diyl diradicals. The observed wavenumbers may offer a target for accurate quantum calculations for singlet diradical species. The second drawback of the calculation is that the upshifts of the CN stretching wavenumber of the azoalkanes (AZ1, AZ2) from the ring-closed species (CP1, CP2) were calculated to be much smaller than the observed shifts. The calculated shifts of dimethoxy- and dimethyl-substituted azoalkanes were both 1 cm−1, while the observed values for AZ1 and AZ2 were 7 and 5 cm−1, respectively. In an attempt to improve the calculated shifts, we employed the B3LYP method with a larger basis set, 6-311+G(d,p), and the B3LYP method with 6-31G(d) taking the solvent (CH2Cl2) into account by using a polarizable continuum model (PCM). The former calculation did not improve the shifts, and the latter gave a slightly better shift, 2 cm−1, for both AZ1 and AZ2. The reason why the shifts of azoalkanes could not be reproduced by the DFT calculations is not clear; however, some solvation effects may have played a significant role (see Tables S3 and S4 in the Supporting Information). To rate the radical character of the diradicals, we compared the observed CN stretching wavenumber of the diradicals with that of a monoradical with a nitrile group whose structure can be regarded as a substructure of the diradicals. 2-(4Cyanophenyl)-propan-2-yl (3) was chosen as the monoradical and 4-tert-butylbenzonitrile (4) as the counterpart of the ringclosed species (Figure 8). The wavenumber shift of the CN stretch mode of the monoradical (3) from the corresponding closed-shell species (4) with a closed-shell electronic

wavenumber shiftb,f

molecule

downshift (cm−1)

a

Table 1. Wavenumbers and Intensities of CN Stretching Vibrations of Radicalsa wavenumberb

method

−20

a

Calculations were performed at the BS-UB3LYP/6-31G(d) level of theory for the diradicals DR1 and DR2, at the UB3LYP/6-31G(d) level of theory for 2-(4-cyanophenyl)-propan-2-yl radical (3), and at the RB3LYP/6-31G(d) level of theory for the closed-shell molecules CP1, CP2, AZ1, AZ2, and 4-t-butylbenzenitrile (4). bWavenumbers and wavenumber shifts are in cm−1. cObserved intensities are in arbitrary units and calculated intensities in D2 Å−2 u−1. dCalculated wavenumbers and intensities of AZ1, AZ2, DR1, DR2, CP1, and CP2 are the weighted mean wavenumbers and sum of intensities of the symmetric and antisymmetric CN stretch modes of each molecule, respectively. eCalculated wavenumbers are scaled with a scaling factor of 0.9494. fWavenumber shifts are calculated from ring-closed compounds (CP1, CP2) for diradicals (DR1, DR2) and from 4 for the mono radical 3.

observed band intensities. The calculated CN wavenumber for each species corresponds to the weighted mean wavenumber of the symmetric and antisymmetric CN stretch modes; the weights were determined by their calculated IR band intensities. The listed intensities are the sum of calculated values for the symmetric and antisymmetric modes. To facilitate the comparison between the observed and calculated values, the calculated wavenumbers were scaled with a scaling factor of 0.9494, which was determined such that the scaled wave3995

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 24350010). In addition, this work was partially supported by a Grant-in-Aid for Science Research on Innovative Areas “Stimuli-responsive Chemical Species” (No. 24109008) from the MEXT, Japan.

Figure 8. Structures of 2-(4-cyanophenyl)-propan-2-yl (3) and 4-tertbutylbenzonitrile (4).

configuration was calculated to be −20 cm−1 by B3LYP (Table 1). The shift of 3 was close to the observed shift (−19 cm−1) of the triplet diradical (DR2); however, it was much larger than that (−10 cm−1) of the singlet diradical (DR1). By comparing the shifts, we conclude that the radical character of DR1 is substantially less than that of DR2, while DR2 has similar radical character as the monoradical, 3. On the basis of the comparison of the CN stretching wavenumber, the radical character of the singlet diradical may be approximately half that of the triplet diradical. We attribute the less significant radical character of the singlet diradical to the contribution from π-single bonding (Figure 1, I) and zwitterionic (Figure 1, III) resonance structures. Although the distance between C1 and C3 is much longer than the ordinary C−C bonds, we may expect some interaction between the two radical electrons if their spins form a singlet combination. The contribution may reflect the facts that the optimized singlet structure of DR1 had a shorter distance (0.02 Å) between the radical carbons (C1−C3) and a longer distance (0.003 Å) between C2 and the methoxy substituents (C2−OMe) as compared to the triplet structure of DR1 (see Table S2 in the Supporting Information).

■

(1) Salem, L.; Rowlaand, C. The Electronic Properties of Diradicals. Angew. Chem., Int. Ed. Engl. 1972, 11, 92−111. (2) Borden, W. T. Diradicals; John Wiley & Sons: New York, 1982. (3) Abe, M.; Ye, J.; Mishima, M. The Chemistry of Localized Singlet 1,3-Diradicals (Biradicals): from Putative Intermediates to Persistent Species and Unusual Molecules with a π-Single Bonded Character. Chem. Soc. Rev. 2012, 41, 3808−3820. (4) Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011−7088. (5) Buchwalter, S. L.; Closs, G. L. An Electron Spin Resonance Study of Matrix Isolated 1,3-Cyclopentadiyl, a Localized 1,3-Carbon Biradical. J. Am. Chem. Soc. 1975, 97, 3857−3857. (6) Xu, J. D.; Hrovat, D. A.; Borden, W. T. Ab Initio Calculations of the Potential Surfaces for the Lowest Singlet and Triplet States of 2,2Difluorocyclopentane-1,3-diyl. The Singlet Diradical Lies Below the Triplet. J. Am. Chem. Soc. 1994, 116, 5425−5427. (7) Adam, W.; Borden, W. T.; Burda, C.; Foster, H.; Heidenfelder, T.; Heubes, M.; Hrovat, D. A.; Kita, F.; Lewis, S. B.; Scheutzow, D.; et al. Transient Spectroscopy of a Derivative of 2,2-Difluoro-1,3diphenylcyclopentane-1,3-diylA Persistent Localized Singlet 1,3Diradical. J. Am. Chem. Soc. 1998, 120, 593−594. (8) Borden, W. T. Diradicals. In Encyclopedia of Computational Chemistry; Schleyer, P. V. R., Allinger, N. L., Clark, T., Gasteiger, J., Kollman, P. A., Schaefer III, H. F., Schreiner, P. R., Eds.; John Wiley & Sons: Chichester, UK, 1998; pp 708−722. (9) Abe, M.; Adam, W.; Nau, W. M. Photochemical Generation and Methanol Trapping of Localized 1,3 and 1,4 Singlet Diradicals Derived from a Spiroepoxy-Substituted Cyclopentane-1,3-diyl. J. Am. Chem. Soc. 1998, 120, 11304−11310. (10) Johnson, W. T. G.; Hrovat, D. A.; Skancke, A.; Borden, W. T. Ab Initio Calculations Find 2,2-Disilylcyclopentane-1,3-diyl Is a Singlet Diradical with a High Barrier to Ring Closure. Theor. Chem. Acc. 1999, 102, 207−225. (11) Abe, M.; Adam, W.; Heidenfelder, T.; Nau, W. M.; Zhang, X. Intramolecular and Intermolecular Reactivity of Localized Singlet Diradicals: The Exceedingly Long-Lived 2,2-Diethoxy-1,3-diphenylcyclopentane-1,3-diyl. J. Am. Chem. Soc. 2000, 122, 2019−2026. (12) Wentrup, C. From Reactive Intermediates to Stable Compounds. Science 2002, 295, 1846−1847. (13) Abe, M.; Adam, W.; Hara, M.; Masanori, H.; Majima, T.; Nojima, M.; Tachibana, K.; Tojo, S. On the Electronic Character of Localized Singlet 2,2-Dimethoxycyclopentane-1,3-diyl Diradicals: Substituent Effects on the Lifetime. J. Am. Chem. Soc. 2002, 124, 6540−6541. (14) Zhang, D. Y.; Hrovat, D. A.; Abe, M.; Borden, W. T. DFT Calculations on the Effects of Para Substituents on the Energy Differences between Singlet and Triplet States of 2,2-Difluoro-1,3diphenylcyclopentane-1,3-diyls. J. Am. Chem. Soc. 2003, 125, 12823− 12828. (15) Jung, Y.; Head-Gordon, M. How Diradicaloid Is a Stable Diradical? ChemPhysChem 2003, 4, 522−525. (16) Abe, M.; Adam, W.; Borden, W. T.; Hattori, M.; Hrovat, D. A.; Nojima, M.; Nozaki, K.; Wirz, J. Effects of Spiroconjugation on the Calculated Singlet-Triplet Energy Gap in 2,2-Dialkoxycyclopentane1,3-diyls and on the Experimental Electronic Absorption Spectra of Singlet 1,3-Diphenyl Derivatives. Assignment of the Lowest-Energy

4. CONCLUSION This paper has reported the first observation of vibrational spectra of cyclopentane-1,3-diyl diradicals. The observed large downshifts of the CN stretching wavenumbers of the target diradicals with two p-cyanophenyl substituents on the two radical carbons compared to corresponding closed-shell pcyanophenylalkanes were interpreted as a consequence of the quinoid resonance structure with cumulative double bonds (CCN•). This structure is possible only when an unpaired electron exists on the carbon at the para-position of the cyano group. This interpretation leads to the conclusion that the radical character of the singlet diradical is less significant than that of the triplet diradical, which may be because the π-single bonding and zwitterionic resonance structures exclusively contribute to the former. The p-cyanophenyl group may be a promising probe for rating the radical character of diradicals. Further studies to confirm the usefulness of the group are required to examine the correlation between the lifetimes and CN stretching wavenumber among cyclopentane-1,3-diyl singlet diradicals with various substituents at C2.

■

ASSOCIATED CONTENT

S Supporting Information *

Further details about the results of the quantum calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (M.A.). *E-mail [email protected] (T.I.). 3996

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997

The Journal of Physical Chemistry B

Article

Electronic Transition of Singlet Cyclopentane-1,3-diyls. J. Am. Chem. Soc. 2004, 126, 574−582. (17) Ma, J.; Ding, Y.; Hattori, K.; Inagaki, S. Theoretical Designs of Singlet Localized 1,3-Diradicals. J. Org. Chem. 2004, 69, 4245−4255. (18) Abe, M.; Kubo, E.; Nozaki, K.; Matsuo, T.; Hayashi, T. An Extremely Long-Lived Singlet 4,4-Dimethoxy-3,5-diphenylpyrazolidine-3,5-diyl Derivative: A Notable Nitrogen-Atom Effect on Intraand Intermolecular Reactivity. Angew. Chem., Int. Ed. 2006, 45, 7828− 7831. (19) Abe, M.; Hattori, M.; Takegami, A.; Masuyama, A.; Hayashi, T.; Seki, S.; Tagawa, S. Experimental Probe for Hyperconjugative Resonance Contribution in Stabilizing the Singlet State of 2,2Dialkoxy-1,3-diyls: Regioselective 1,2-Oxygen Migration. J. Am. Chem. Soc. 2006, 128, 8008−8014. (20) Nakamura, T.; Takegami, A.; Abe, M. Generation and Intermolecular Trapping of 1,2-Diaza-4-silacyclopentane-3,5-diyls in the Denitrogenation of 2,3,5,6-Tetraaza-7-silabicyclo[2.2.1]hept-2-ene: An Experimental and Computational Study. J. Org. Chem. 2010, 75, 1956−1960. (21) Ye, J.; Fujiwara, Y.; Abe, M. Substituent Effect on the Energy Barrier for σ-Bond Formation from π-Single-Bonded Species, Singlet 2,2-dialkoxycyclopentane-1,3-diyls. Beilstein J. Org. Chem. 2013, 9, 925−933. (22) Nakagaki, T.; Sakai, T.; Mizuta, T.; Fujiwara, Y.; Abe, M. Kinetic Stabilization and Reactivity of π Single-Bonded Species: Effect of the Alkoxy Group on the Lifetime of Singlet 2,2-Dialkoxy-1,3diphenyloctahydropentalene-1,3-diyls. Chem. Eur. J. 2013, 19, 10395−10404. (23) Kita, F.; Adam, W.; Jordan, P.; Nau, W. M.; Wirz, J. 1,3Cyclopentanediyl Diradicals: Substituent and Temperature Dependence of Triplet-Singlet Intersystem Crossing. J. Am. Chem. Soc. 1999, 121, 9265−9275. (24) Iwata, K.; Hamaguchi, H. Construction of a Versatile Microsecond Time-Resolved Infrared Spectrometer. Appl. Spectrosc. 1990, 44, 1431−1437. (25) Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Nanosecond Time-Resolved Infrared Spectroscopy with a Dispersive Scanning Spectrometer. Appl. Spectrosc. 1994, 48, 684−690. (26) Yamakata, A.; Ishibashi, T.; Onishi, H. Time-Resolved Infrared Absorption Spectroscopy of Photogenerated Electrons in Platinized TiO2 Particles. Chem. Phys. Lett. 2001, 333, 271−277. (27) Maeda, A.; Ishibashi, T. Time-Resolved IR Observation of a Photocatalytic Reaction of Pivalic Acid on Platinized Titanium Dioxide. Chem. Phys. 2013, 419, 167−171. (28) Gaussian 09, Revision B.01: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian, Inc.: Wallingford, CT, 2009. (29) Yamaguchi, K. The Electronic Structures of Biradicals in the Unrestricted Hartree-Fock Approximation. Chem. Phys. Lett. 1975, 33, 330−335. (30) Noodleman, L. Valence Bond Description of Antiferromagnetic Coupling in Transition Metal Dimers. J. Chem. Phys. 1981, 74, 5737− 5743. (31) Neese, F. Definition of Corresponding Orbitals and the Diradical Character in Broken Symmetry DFT Calculations on Spin Coupled Systems. J. Phys. Chem. Solids 2004, 65, 781−785. (32) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (33) On the basis of the calculated energy difference (0.66 kJ/mol) between the triplet and singlet states of DR2, the population ratio of the singlet to the triplet at 293 K may be calculated to be (1/3) exp(−55/204) ≈ 0.25. Then, the contribution from the singlet DR2 may apparently broaden the bandwidth of the CN stretch band of DR2 because the CN stretch wavenumber of the singlet DR2 is expected to be similar to that of the observed singlet DR1, 2218 cm−1. However, no indication of such broadening was confirmed on the

observed CN stretch band of DR2; the bandwidths (fwhm) of DR1 and DR2 were 16.0 and 15.9 cm−1, respectively. This suggests that the UB3LYP calculation underestimated the stability of the triplet state over the singlet state for DR2, and there was virtually no contribution from the singlet DR2 on the observed TRIR spectra.

3997

dx.doi.org/10.1021/jp500636j | J. Phys. Chem. B 2014, 118, 3991−3997