Solvent Induced Transformations of nâÏ* Absorption in Formaldehyde

Article pubs.acs.org/JPCA

Solvent Induced Transformations of n−π* Absorption in Formaldehyde, Acetaldehyde, and Acetone Indrek Renge* Institute of Physics, University of Tartu, Ravila Street 14c, EE50411 Tartu, Estonia ABSTRACT: Absorption spectra of formaldehyde (FA), acetaldehyde (AA), and acetone are compared in the vapor phase, nonpolar, and polar solutions at 295 K. The vibronic n−π* transition of carbonyl chromophore is mainly composed of the overtones of >CO stretching vibration. A new phenomenon is observed in liquid solutions, consisting of a relative increase of Franck−Condon factors for the second and third harmonics in FA, and the second to fourth replica in AA, with respect to the gas phase. In AA and acetone with poorly resolved vibronic structure, the redistribution of intensities produces a false “solvent shift” of the band maximum between the vapor and nonpolar liquid phase by −250 ± 50 cm−1. Modification in vibronic coupling can also explain unusual narrowing of the band contour in the solution, reported earlier for acetone (Renge, I. J. Phys. Chem. A 2009, 113, 10678). No detectable shift occurs as a function of solvent polarizability (refractive index function (n2 − 1)/(n2 + 2)) in n-alkanes for FA, AA, and acetone, as well as for cyclopentanone and camphor. Incidentally, the bathochromic dispersive shift is almost exactly compensated by a hypsochromic induction shift. The latter is due to the diminishing dipole moment in the excited state of the carbonyl chromophore. Differences in polarizability α and dipole moments μ were estimated for FA (Δα = 0.33 ± 0.1 Å3), AA (Δμ = −1.05 ± 0.2 D, Δα = 0.5 ± 0.2 Å3), and acetone (Δμ = −1.3 ± 0.2 D, Δα = 0.65 ± 0.2 Å3). The increase of α by ∼10% upon excitation is plausible for a weak n−π* transition. By contrast, near doubling of α in the upper state has been reported recently for several ketones, with Δα reaching 10 Å3 (Catalán, J.; Catalán, J. P. Phys. Chem. Chem. Phys. 2011, 13, 4072). Empirical partitioning of solvent shifts into repulsive−dispersive, induction, dipole−dipole, and hydrogen bonding contributions was proposed to serve as a benchmark in computer chemical calculations. ketones.5 Here we will confirm our earlier result about the independence of band position on the Lorentz−Lorenz function ϕ(n2) of refractive index n (ϕ(n2) = (n2 − 1)/(n2 + 2)) in alkane solvents.4 The lack of ϕ(n2) dependent shift was ascribed to mutual cancellation of repulsive−dispersive and induction shift components, the latter being hypsochromic,4 because the solute dipole moment in the excited state μe is much less than that in the ground state (μg).38−40 The induction shift is proportional to μg2 − μe2, and can be calculated, if dipole moments are known. Subsequently, Δα is estimated from the dispersive red shift of equal magnitude, but of opposite sign.4 We will discuss also the possibility of similar compensation phenomena between the repulsive and dispersive shift components, in terms of Lennard-Jones model of pair potentials.41 An absolute solvent shift between the gas and the liquid phase is of considerable interest for verification of solvent shift calculations.11−30 The magnitude of absolute shift has been highly controversial in aliphatic ketones, ranging from a red shift of −300 cm−1 for acetone to a 450 cm−1 blue shift for 4heptanone in n-heptane solutions.37 It will be shown that spectra of the vapor and solution cannot be directly compared, because of redistribution of overtone intensities between different phases. A slight enhancement of Franck−Condon

1. INTRODUCTION Carbonyl compounds play a prominent role in spectroscopy, photochemistry, and chemical theory. The carbonyl chromophore is a potentially versatile optical probe to intermolecular interactions by means of absorption, IR, and Raman spectroscopy. The optical n−π* transition and the >CO double bond stretching are both very sensitive with respect to donor− acceptor complexing, hydrogen bonding, and internal fields.1−3 Large dipole moment change renders the n−π* band highly sensitive to polarity of the environment.4,5 Acetone has been applied for monitoring properties of hypercritical water and clustering of solvent around the probe molecule.6,7 Carbonyl functionalities in chlorophylls have proved useful as resonance Raman markers of pigment protein interactions.8,9 Due to its small size, methanal (formaldehyde, FA) appears particularly well suited as a local probe.10 Solvent effects on the n−π* transition in FA11−22 and acetone19−30 have been extensively treated in computer chemical calculations. Very few solvent shift studies were performed on the simplest carbonyls, FA,31,32 and acetaldehyde (AA).33,34 Experimental work on acetone absorption is quite old and inconsistent.1,33−37 Spectral shifts of irregular n−π* bands cannot be defined in a straightforward fashion, because broadening modulates the location of peak maxima. Recent, rather extensive papers have exacerbated the confusion about solvent shifts.4,5 Contradictory molecular polarizability differences Δα were reported for acetone, extending from 0.6 ± 0.2 Å3 4 to 7.38 Å3, and even larger (7.65−12.9 Å3) for higher © 2015 American Chemical Society

Received: April 17, 2015 Revised: July 2, 2015 Published: July 16, 2015 8599

DOI: 10.1021/acs.jpca.5b03695 J. Phys. Chem. A 2015, 119, 8599−8610

Article

The Journal of Physical Chemistry A factors for the lower harmonics has a strong influence on apparent band shifts in solution. The remarkable change in vibronic intensities could be ascribed to a decrease in displacement of potential minima along the >CO stretching coordinate in the condensed phase. Further, assuming independence of shift on refractive index in all solvents, not only n-alkanes, the solvent polarity effect can easily be obtained. Because the dipole moments of FA μg and μe are known for both states (2.332 ± 0.002 D40 and 1.56 ± 0.07 D,38 respectively), the empirical expression for the dipolar shift, depending on μg(μg − μe) can be calibrated and applied for estimation of μe of AA and acetone. Spectral shifts show a meaningful dependence on static dielectric constant ε only in aliphatic, aprotic, and monofunctional solvents.4,42 The lack of many μe values, as well as polarizabilities in the excited state can be associated with difficulties of Stark effect measurements on very broad, vibronically crowded spectra of alkyl derivatives of CH2O. The significance of a detailed characterization and improved understanding of solvatochromism in carbonyl compounds is 2fold: to provide the reference data for molecular probing purposes and the much needed benchmarks for computer chemical calculations.11−30

Figure 1. Gas (vapor) phase absorption spectra of formaldehyde, acetaldehyde, and acetone at ambient temperature and pressure. The slit width was 1 nm for FA and 2 nm for AA and acetone. Positions of purely electronic 0−0 origins are indicated by bars (refs 38, 47, and 48).

2. EXPERIMENTAL SECTION Acetaldehyde, paraformaldehyde (95% (CH2O)n, mp 163−165 °C (with decomposition)), and solvents of high purity were purchased from Aldrich. Liquids were dehydrated by Drierite and 3A molecular sieves when necessary. Absorption spectra were recorded on a JASCO V-570 UV/vis/NIR spectrophotometer at 295 K. Spectra of AA and acetone vapors, and gaseous CH2O were measured in a homemade, cylindrical silica cell of 40 mm path length whose side arm is closed with a polyethylene cap. The instrument was calibrated with 287.15, 278.2, and 241.15 nm lines of 0.25 M HoCl3 in 0.1 M aqueous HCl43 and found reproducible within 0.1 nm, with systematic error not exceeding 0.5 nm. Formaldehyde was prepared by slight heating on a gas flame of paraformaldehyde powder contained in a round-bottom flask and bubbled through liquid directly in a 10 mm cell, prior to running the spectrum. Most solutions were stable in the course of measurement (several minutes), except for water and alcohols.32,34

Figure 2. Absorption spectra of formaldehyde in gas and normal alkane solutions. Spectra in n-alkanes C5,6,7,10,16 are indistinguishable. The intensity of 0−2 and 0−3 bands is enhanced in the condensed phase. The 0−0 band is subject to a small pseudoshift as a result of broadening. The slit width was 2 nm.

3. RESULTS Solvent or temperature induced broadening of composite spectral contours seriously complicates the quantitative determination of the shifts. The spectra of CH2O are well resolved in the gas phase and n-alkanes (Figures 1 and 2). The structure is gradually lost with increasing solvent polarity (Figures 3 and 4). Broadening of overlapping lines can cause spurious shifts of maximum positions (Figure 2). Incidentally, the spectrum is quite symmetrical with respect to central peaks marked as 0−3 and 0−4. The average wave numbers of the two or, alternatively, four (from 0−2 to 0−5) central peaks coincide within 10 cm−1, so that the maximum can be ascertained even in the most polar solvents. In the case of broad (>5000 cm−1), humpy band contours the determination of small shifts with reasonable accuracy (50 cm−1) needs special care to avoid pseudoshifts. In acetaldehyde (AA) the vibronic structure is better resolved than in acetone, and the contour is almost symmetrical (Figure 1). The structure is effectively smoothed out, if spectra are recorded

with a 10 nm slit width, and the central position is fixed at about 0.98 of relative height of the band. Finally, the n−π* absorption band of acetone shows barely resolved vibronic structure in either the vapor phase or apolar solutions. A simple procedure referred to as “band-halving” was adopted in our previous work.4 The frequency dependence of the midpoint positions of normalized spectra is approximately linear at relative heights from 0.6 to 0.9. Interpolation to a somewhat arbitrary relative height of 0.85 is used here to define shifts between different solvents. With the aid of these seemingly arbitrary tricks, the solvent shifts can be obtained with reproducibility ±20 cm−1 (Table 1). Unfortunately, the procedure used for determination of shifts has not been specified in sufficient detail in many older papers, so that considerable uncertainty remained in solvatochromic behavior of n−π* bands in aldehydes32−34 and ketones.1,33−37 8600


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The Journal of Physical Chemistry A

increases from C5 to C16 (Figure 2).45 However, perfluorinated alkanes behave as less interacting and show spectral shapes that are intermediate between the vapor and alkanes (Figures 5 and 6). In qualitative terms, a modification in Franck−Condon coupling constants for the harmonics of carbonyl stretching mode in the excited state (1120−1230 cm−1)10,46 may be associated with a cage effect in condensed phase environment. Substantial lengthening of >CO bond in the excited state (by ∼10%)10,46 translates into a horizontal displacement of potential energy curve that means enhanced linear Franck− Condon coupling strength. The solvent−solute repulsion in a cavity would hinder the bond stretching, thus promoting lower harmonics as illustrated in Figure 7. The expected increase of vibronic frequency (quadratic Franck−Condon coupling) can be difficult to detect, because of poor spectral resolution in the condensed phase. Positions of adiabatic, 0−0 transitions recorded in FA gas38 or cold molecular beams47−49 are denoted by bars in Figures 1, 5, and 6. The vapor absorption is negligible at these frequencies. The band maxima of FA, AA, and acetone (as defined in this work) are shifted from the 0−0 origins by 4100, 5000, and 5800 cm−1 that correspond to 3.7, 4.2, and 4.7 vibrational quanta of >CO stretching mode, respectively (Table 2). Bandwidth Narrowing in Acetaldehyde and Acetone Absorption. As compared to the situation in the gas phase, spectral resolution gets worse in solutions as a result of broadening. The well resolved structure in CH 2 O is progressively lost with increasing polarity (Figure 4). This contrasts with overall band narrowing by ∼400 cm−1 reported for acetone without proper explanation in our previous work.4 The narrowing effect in acetone is actually even larger, because the vapor spectrum was distorted in that work. The full width at half-maximum (fwhm) shrinks from 6800 ± 30 cm−1 in the vapor to 6560 ± 30 cm−1 in n-alkanes, and further to 6300 ± 30 cm−1 with increasing polarity in acetonitrile and water (narrowing by 500 cm−1). A very similar phenomenon is documented in AA spectra recorded with 10 nm resolution, consisting of a decrease from 6520 ± 50 cm−1 of fwhm in the vapor phase to 6300 cm−1 in n-alkanes, and further to 6200 cm−1 in nitriles and DMSO (narrowing by 320 cm−1). As in case of acetone,41 the fwhm of AA is roughly correlated with dielectric constant as a possible measure of liquid cohesion energy45 (Figure 8). However, in highly polar and protic methanol (no. 41) and water (no. 42) the fwhm is enhanced again, probably as a result of disorder in hydrogen bonding. The narrowing can be attributed to compression of Franck− Condon profile of a multiphonon envelope between the vapor and solution phase. The reduced bandwidth is closely related to redistribution of overtone intensities discovered in this study. The effect is strong enough to compensate the increase of net homogeneous and inhomogeneous broadening in polar environment, as manifested clearly by the gradual loss of vibronic structure with increasing solvent polarity (Figure 4). 4.2. Nonspecific Solvation. Solvent Polarizability Dependent Shifts. Dielectric permittivity of apolar liquids is solely due to electronic polarization, as the Maxwell relation (ε = n2) is almost exactly obeyed (Table 1). The plot of transition energy νmax against the Lorentz−Lorenz function ϕ(n2) is usually perfectly linear in n-alkanes:50,51

Figure 3. Absorption spectra of FA in gas and apolar solutions. Broadening causes progressive loss of structure in gas < C6 < CCl4 < dioxane.

Figure 4. Absorption spectra of FA in gas and polar solutions. Broadening causes loss of structure with increasing polarity in diethyl ether < methyl acetate < acetonitrile.

4. DISCUSSION 4.1. Vibronic Coupling. Change of Franck−Condon Profile between the Vapor and Liquid. It is usually assumed that vibronic coupling to intramolecular modes is the same in the gas and solution phase. A rare occasion when vibronic intensities are subject to a change is documented here. This would also result in a pseudoshift of spectral peak maxima. The effect was first noticed by comparing the spectra of AA in vapor and n-heptane (C7) solution. The enhancement of the 0−3 and 0−4 harmonics of carbonyl stretching mode at the expense of higher, 0−6 and 0−7, replicas is obvious in the condensed phase (Figure 5). The spectra run with 10 nm resolution to smooth out the structure show a ∼−300 cm−1 bathochromic shift in alkanes that is not related to solvent shift in the usual sense (not shown). The phenomenon can also be seen in well structured absorption of FA (Figure 2) and recognized in almost the featureless contour of acetone (Figure 6). The vibronic structure remains constant in n-alkanes (Cn), independent of cohesion energy, or “internal pressure” that

νmax = ν0 + pϕ(n2) 8601

(1) DOI: 10.1021/acs.jpca.5b03695 J. Phys. Chem. A 2015, 119, 8599−8610

Article


Table 1. Properties of Solvents and the Shifts of n−π* Absorption Band in Formaldehyde (FA), Acetaldehyde (AA), and Acetone at 295 Ka Δν (cm−1) no.

ε

solvent

1

vapor

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

n-pentane n-hexane n-heptane n-octane n-nonane n-decane n-undecane n-dodecane n-hexadecane perfluoro-n-heptane perfluoro-n-octane cyclohexane hexafluorobenzene dioxane carbon tetrachloride benzene

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

di-n-butyl ether diethyl ether butyl acetate methyl butanoate propyl acetate methyl acetate 1-bromobutane tetrahydrofuran chlorocyclohexane tetrahydrothiophene bromoethane (±)-propylene oxide acetone butyronitrile propionitrile acetonitrile N,N-dimethylformamide N,N-dimethylacetamide γ-butyrolactone tetramethylene sulfoxide dimethyl sulfoxide propylene carbonate

40 41 42

hexafluoro-2-propanol methanol water

ε′

1

nD

20

1 1 Apolar Solvents (ε < 1.15n2) 1.8371 1 1.3575 1.8865 1 1.375 1.9209 1 1.388 1.948 1 1.3974 1.9722 1 1.4058 1.9853 1 1.412 1.9972 1 1.4164 2.0120 1 1.421 2.0460 1 1.434 1.76 1.13 1.261 1.81 1.12 1.284 2.0243 1 1.426 2.029(25) 1.09 1.3777 2.2189 1.12 1.422 2.2379 1.06 1.46 2.2825 1.02 1.501 Polar Monofunctional Aliphatic Solvents 3.0830 1.78 1.398 4.2666 2.81 1.352 5.07 3.3 1.394 5.50 3.6 1.386 5.62 3.7 1.384 6.94 4.8 1.3614 7.16 4.6 1.439 7.47 5 1.406 7.60 4.8 1.463 8.61 5.3 1.504 9.59 6.5 1.424 9.9calc 7.1 1.366 21.01 15.8 1.359 24.83 18.5 1.383 29.7 22.5 1.366 36.64 28.3 1.344 38.25 27.7 1.43 38.30 27.6 1.438 39.0 28.1 1.437 42.84 29.1 1.520 46.71 33 1.479 66.14 49 1.421 Protic Solvents 16.70 11.4 1.46 33.0 25.8 1.3288 80.100 63 1.33336

FA

AA

acetone

0 ± 20

(300)b

(200)b

10 20 10 20 10 20 0 10 20 70 90 0 280 240 110 200

−10 0 −10

30 0 0

0 −40 30 20 −40 240 220 −50 130 350 −150 −80

0 −10 0 0 10 180 190

50 70 170 180 190 290 140 130 130 200 220 300 360 480 270 240 440 410 510

20 150 230 230 290 380 40 280 50 −110 120 400 310 360 440 290

300 −240

30 160 190 190 310 10 250

300 400 270 290 450

410 230 320 540

350 380

2050 650 1630

2300 950 1800

Solvent shifts in AA and acetone are given with respect to average value in n-alkanes; ε, dielectric permittivity; ε′, dipolar component of ε from eq 7; nD20, refractive index for Na D line (from refs 40, 44, and 45). bPseudoshift due to vibronic intensity redistribution.

a

moment. The dispersive Δνdisp and induction Δνind components of solvents shifts were expressed as4,52

The spectra of FA (Figure 2), AA, and ketones are virtually identical in Cn, showing no shift with changing solvent refractive index, so that the slope p = 0 ± 20 cm−1 (Figure 9). Besides reconsidering acetone,4 we checked also cyclopentanone and camphor in n-alkanes C5 to C16 for the lack of any detectable spectral change (data not shown). The absence of solvent shift as a function of solvent polarizability ϕ(n2) means that bathochromic dispersive shift must be compensated by hypsochromic induction shift, due to decreasing dipole

Δνdisp [cm−1] = −5.5 × 104ΔαMW −1ϕ(n2)

(2)

Δνind [cm−1] = 6.3 × 103(μg 2 − μe 2 )MW −1ϕ(n2)

(3) −30

where μg and μe are in Debye units (1 D = 3.336 × 10 C m), ϕ(n2) = (n2 − 1)/(n2 + 2). The solute radius cubed a3 (in Å3 units) is assumed to be constant in both ground and excited states in most solvent shift 8602


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Figure 7. Modulation of linear vibronic coupling strength by repulsive interactions in the condensed phase. The vertical, Franck−Condon transition is illustrated for the free and the “caged” chromophore to occur to the seventh and third upper state >CO stretching level, respectively.

Figure 5. Absorption spectra of acetaldehyde in the vapor, nperfluoroheptane, and n-heptane. Intensities of 0−4 and 0−6 bands are inverted between the vapor phase and C7 (inset).

are very pronounced in aldehydes and acetone (Figure 9). Large dipole moments are capable of creating reaction fields in the presence of solvent bond dipoles or multipoles.61,62 Large blue shifts in dioxane (240, 350, and 300 cm−1 for FA, AA, and acetone, respectively) can be due to group dipole moment of the O< fragment, rather than the large overall quadrupole moment of dioxane.59,61,63 Dioxane behaves often as moderately “polar” medium, despite being nominally non(di)polar, with n2 equal to 2.02 and ε 2.22. The influence of quadrupolar benzene and octupolar CCl4 is less clear, because the FA spectrum is blue-shifted by 200 and 110 cm−1, respectively. By contrast, AA has small red shifts of −80 cm−1 in C6H6, and −150 cm−1 in CCl4 (Table 1). The relative blue shifts in perfluorinated solvents and red shifts CCl4 could be rationalized in terms of nonuniform polarizability density. The latter was defined4,51,64 as a ratio of group polarizability after Vogel αgp65 to van der Waals radius rw cubed (after Bondi).66 In aliphatic compounds the atoms H, C, N, and O possess similar αgp/rw3 values of 0.25 ± 0.05. The polarizability density of F and Cl atoms is very different, amounting to 0.10 and 0.43, respectively. As a consequence, the dispersive red shift is less than predicted from ϕ(n2) in perfluorinated solvents but is underestimated for CCl4. The effect can be enhanced, if dispersion has a shorter interaction radius than induction (polarization of solvent by solute dipole) (see below). The study in apolar solvents shows that, because of small size of carbonyl chromophore, CH2O is highly sensitive with respect to atomic-scale density fluctuations, acting via either the charges (group dipole and multipolar moments) or polarizability density. Estimation of Polarizability Difference Δα. Apparent lack of influence of refractive index on spectra in Cn can be rationalized in terms of cancellation of dispersive (eq 2) and induction shifts (eq 3), so that the net slope p of the Lorentz− Lorenz function dependence is zero:

Figure 6. Absorption spectra of acetone in vapor, n-C7F16, and nC7H16. The apparent red shift in C7 is ascribed to vibronic intensity redistribution in the liquid phase (inset), similar to that in FA and AA.

theories.53−59 Incidentally, a3 can be replaced with the molecular weight MW of solute.50 This simplification was first derived by calibration of ϕ(n2) dependencies of absorption band maxima of nonpolar polyarenes with Δα measured directly by means of the quadratic Stark effect.50 Molecular weight as a measure of the cavity size was extended to induction (eq 3) and to dipole−dipole interactions (eq 6).42 In general, the (equilibrium) interaction radii may vary between the ground and the excited state, and also for different types of interactions. These effects will be elaborated in separate chapters below. Remarkable deviations from the ϕ(n2) dependence occur in apolar liquids other than Cn. The so-called dioxane and perfluoro effects, as well as the tetrachloromethane anomaly are well documented in the studies of solvatochromism51,59,60 and

p = − 5.5 × 104ΔαMW −1 + 6.3 × 103(μg 2 − μe 2 )MW −1 = 0 (4) 8603


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The Journal of Physical Chemistry A Table 2. Ground and Excited State Properties of Formaldehyde (FA), Acetaldehyde (AA), and Acetone property

symbol/expression

unit

FA

AA

acetone

adiabatic energya band max. in vaporb adiabatic-vertical energy diff >CO stretch in S1c >CO harmonic no. band maximum in Cnb molecular weightd ground state polarizabilityd excited state polarizability polarizability diff ionization potentiald ground state dipole momentd excited state dipole moment dipole moment diff calcd induction coeff measd dip.-dip. coefff interceptf no. of solventsf regression coefff

ν00 νmax0 νmax0 − ν00 νvib(S1) (νmax0 − ν00)/νvib(S1) νmax MW αg αe Δα I μg μe −Δμ 6.3 × 103(μg2 − μe2)/MW y′

cm−1 cm−1 cm−1 cm−1 cm−1 cm−1 Da Å3 Å3 Å3 eV D D D cm−1 cm−1 cm−1

28201 32300 4100 1120 3.7 32300 ± 20 30.03 2.8 3.1 0.33 ± 0.1 10.88 ± 0.01 2.332 ± 0.002 1.56 ± 0.07e 0.77 ± 0.07 630 395 ± 30 0 ± 20 30 0.92

29771 34750 4980 1173 4.2 34450 ± 50 44.05 4.6 5.1 0.5 ± 0.2 10.229 ± 0.007 2.750 ± 0.006 1.7 ± 0.2 1.05 ± 0.2 670 420 ± 45 −10 ± 30 29 0.87

30435 36250 5800 1226 4.7 36050 ± 50 58.08 6.4 7.0 0.65 ± 0.2 9.703 ± 0.006 2.88 ± 0.03 1.6 ± 0.2 1.3 ± 0.2 620 410 ± 25 0 ± 10 22 0.97

N |r|

0−0 transition energy from refs 38 and 47−49. bAs defined in this work. cIn refs 46 and 48. dIn ref 40. eIn ref 38. fLinear fitting parameters to eq 8; see Figure 10.

a

Figure 8. Dependence absorption bandwidth (fwhm) on dielectric permittivity for AA. Bands tend to narrow with increasing solvent polarity (and cohesion energy), except for protic methanol (no. 41) and water (no. 42). Spectra were recorded with 10 nm slit width to smooth out structure. Numbers correspond to solvents in Table 1.

Figure 9. Dependence of spectral shifts on Lorentz−Lorenz function in apolar solvents for FA (lower panel), AA, and acetone (upper panel). The solvent polarizability dependence is absent in alkanes. A relative blue shifts occur in perfluorinated solvents (nos. 11, 12, 14) and dioxane (no. 15). Shifts in CCl4 (no. 16) and benzene (no. 17) can have different signs. Points corresponding to the same solvent are connected by lines.

Hence Δα [Å3] = 0.11(μg 2 − μe 2 )

The Δα calculated on the basis of solvatochromism could be subject to a considerable uncertainty, because the carbonyl chromophores are much smaller than aromatic molecules used for calibration of eq 2.50 Still the CASSCF and CASPT2 methods yielded a similar adiabatic dipole polarizability change Δα for the acetone 1n−π* state of 0.9 ± 0.1 Å3 (Table 7 in ref 68, it is not clear what the authors mean by Δα for the 1X ground state in Table 5). Substituent Effect as Intramolecular Solvent Effect. Very weak adiabatic transitions occur at 28201 cm−1 (354.6 nm) for FA in the gas phase,38 and at 29771 and 30435 cm−1 for AA and acetone in cold jets,47−49 respectively (Table 2, marked by bars in Figure 1). The methyl group raises the 0−0 transition energy by 1600 cm−1 in AA, whereas the second CH3 in

(5)

A summary of dipole moments (ref 40, pp 9−52) and polarizability in the ground state (ref 40, p. 10−199) is given in Table 2, together with Δα, calculated from eq 5, and the dipole moment of the excited state (see next section). A rather uniform change of polarizability in the excited state by ∼10% over the ground state value is found. The Δα increases by 0.33, 0.5, and 0.65 Å3, in parallel with αg equal to 2.8, 4.6, and 6.4 Å3 for AF, AA, and acetone, respectively. Near doubling of polarizability in the upper state has been reported recently for several ketones, with Δα as large as 7−13 Å3.5 The Δα is apparently strongly overestimated by equations derived by Abe.5,67 8604


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The Journal of Physical Chemistry A acetone adds much less (660 cm−1) to zero phonon frequency. In parallel, the HOMO level raises upon methyl substitution by ∼104 cm−1, because the ionization energy decreases from 10.9 to 9.7 eV40 (Table 2). Hence, the LUMO level ought to be upshifted (and the electron affinity decreased) even more, because the 0−0 energy increases in the sequence of CH2CO > MeCHO > Me2CO (Me = CH3).69 Formally, such behavior is at odds with the expected dispersive red shift by polarizable methyl group(s). However, with growing bulkiness and branching of alkyl substituents the absorption maxima in aliphatic ketones start shifting bathochromically.33,36,37 The spectral red shift between Me2CO and di-tert-butylketone was reported to lie in the interval from −2200 cm−1 35,36 to −2700 cm−1 33 in C6 or C7 solution. A remarkable linear correlation was found between the shift and the sum of steric constants of substituents −ΣEs° (see Figure 7 in ref 4). Substituents were regarded as polarizable solvent particles, though located very close to chromophore.4 We speculated that negative dispersive shift and positive induction shift may not cancel in this case, because the effective radii for these two interactions differ. The short-range dispersion interaction causes a stronger red shift that is not fully compensated by long-reaching polarization (=induction). A smaller interaction radius for the dispersive shift would enhance deviations in perfluorinated solvents and CCl4 that were explained previously in terms of very different polarizability densities of F and Cl atoms, as compared to C, H, O, and N. For AA and acetone the red shift is less in n-C7F16 or n-C8F18 and is increased in CCl4 (Figure 9, Table 1). However, the very small chromophore of FA suffers a small blue shift in CCl4, ascribable to multipolar interactions. Such qualitative reasoning has a certain appeal of “explaining” and “understanding” complex phenomena. Computer chemistry is expected to render support or falsify these largely intuitive considerations. Solvent Polarity Dependent Shift. Polar solvents are defined as those with ε exceeding 1.15n2 (Table 1). Vapor is not a proper reference state to solvent shift in AA and acetone, because vibronic intensity redistribution takes place between the two phases. Instead, the intercept of eq 1 ν0 is chosen as a reference. The ν0 coincides with the band positions in n-alkanes within 50 cm−1. The interaction between the solute and solvent dipoles is treated as an intrinsic electrochromic shift in the reaction field of a chromophore. A simplified expression for the ε dependence has been proposed,42,52 basing on works by Bakhshiev53 and others:54−58 −1

4

−1

Δνdip [cm−1] = (6.3 ± 0.5) × 103μg (μg − μe cos γ )MW −1ϕ(ε′) = y′(ε′ − 1)/(ε′ + 2)

The solvent refractive index n, dielectric constant ε, and its effective value ε′ are collected in Table 1. It should be kept in mind that the bulk dielectric constant ε effectively describes the interaction between the dipoles only in a restricted set of liquids. The well behaving solvents are preferably aliphatic and monofunctional, including ethers, esters, halogenides, ketones, nitriles, amides, and sulfoxides that can contain methyl, ethyl, and nonbranched propyl and butyl substituents, as well as alicyclic derivatives (Table 1).4,42,74 As already discussed above, the prominent and universal “dioxane effect”59,61,63 points to the locality of electrostatic interactions that is not compatible with the point dipole and continuum dielectrics concepts. The net molecular dipole moment (and the related ε) becomes irrelevant for multifunctional solvents such as dioxane or 1,2-dichloroethane. Deviating data with respect of a “well behaving” joint set of n-alkanes and monofunctional polar solvents can reveal meaningful details of solute−solvent interactions.61,73 Estimation of μe. Equation 8 was calibrated by using the available dipole moments of FA in the ground (μg = 2.332 ± 0.002 D)40 and the excited state (μe = 1.56 ± 0.07 D).38 The slopes of eq 8 y′ are remarkably similar for FA, AA, and acetone (y′ ∼ 400 cm−1) (Figure 10). The linear fit is more reliable for

Figure 10. Dependence of spectral shifts of FA, AA, and acetone on solvent dipolarity function ϕ(ε′) for a joint set of alkanes and monofunctional liquids. Linear fits eq 8 have identical slopes (see regression data in Table 2, the most outlaying points nos. 24 and 27 were omitted).

2

Δνdip [cm ] = 1.26 × 10 μg (μg − μe cos γ )MW [ϕ(ε) − ϕ(n )] (6)

where γ is the angle between the solute dipole moments in different states. As emphasized already by Gerhold and Miller70 and others,71,72 the effective dielectric constant ε′ for a hypothetical, nonpolarizable system of dipoles cannot be calculated as a difference ϕ(ε) − ϕ(n2). To find ε′, the quadratic eq 6 in ref 70 has been solved:73 ε′ = (d + 1)/4 + ((d + 1)2 /16 + 0.5)0.5

(8)

the larger acetone molecule (see the regression parameters in Table 2). The scatter of data points is substantial in FA, and particularly bad in AA. Using the μg40 and MW values from Table 2, one finds that the excited state dipole moment μe is practically independent of methyl substitution, μe = 1.6 ± 0.2 D. μg increases slightly from 2.332 D for FA to 2.88 D for acetone.40 It follows from the constant sensitivity to polarity y′ and the increasing molecular size that the difference −Δμ should increase from 0.77 D38,40 in FA to 1.05 in AA to 1.3 D in acetone. Incidentally, the adiabatic dipole moment of the 1 n−π* state of acetone, calculated by means of CASSCF and CASPT2 methods, lies between 1.67 and 1.87 D (Table 7 in ref 68), which are very close to our μe of 1.6 D.

(7)

where d = a/b, a = ϕ(ε) − ϕ(n2), and b = ε/(ε + 2)(2ε + 1). The polarity scale in terms of (ε′ − 1)/(ε′ + 2) has a larger variation range, from 0 to 1, so eq 6 is changed. For our small chromophores, it is reasonable to use a calibration with the available dipole moment values of FA38,40 (see next section): 8605


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The Journal of Physical Chemistry A U = ε[(σ /r )12 − 2(σ /r )6 ]

4.3. Specific Solvation in Protic Solvents. Conspicuous blue shift of n−π* transition in water is well-known,1,4,5,33−37 and treated in numerous QM calculations of carbonyl chromophore.11−30,75 The hypsochromic shift in AA absorption grows fast from methanol (650 cm−1) to water (1630 cm−1) to hexafluoro-2-propanol (2050 cm−1). In acetone the respective shifts are larger by ∼250 ± 50 cm−1 (Table 1). Hydrogen bonding to acetone appears to be stronger, in accordance with electron donating property of the methyl substituent. This effect has been noticed already by Ten-no et al.20 and reproduced by RISM-SCF calculations. The solution of formaldehyde in water and alcohols is not stable,32 so the solvent shift cannot be measured. More than 99% of aqueous FA exists as the gem-diol, whereas for larger aldehydes the amount of free and hydrated form is roughly equal.34 If it is assumed that the (hypothetic) shift of FA in H2O is diminished further by 300−400 cm−1 to 1200 ± 200 cm−1, it would be in good agreement with several older calculations.20−22 More than a dozen of computer chemical papers are dealing with the FA−water problem,11−22 several of them containing small overviews of earlier results.16−18 Most calculated shifts in water are probably overestimated, and the frustrating lack of convergence over the decades of theoretical work may be largely due to missing reference data. It is not always clear whether the pure hydrogen bonding effect is being calculated or if the shift also includes the dipole− dipole blue shift. Within approximation of additivity of “interactions”, the total shift in hydroxylic solvents is a superposition of the dipole−dipole shift Δνdip, occurring, e.g., in aprotic acetonitrile, and the H-bonding effect.76 To estimate the “pure” specific solvation contribution, a flat Δνdip ∼400 cm−1 is subtracted. As already mentioned, the “pure” Hbonding effect ΔνH is always less for AA, as compared to acetone, in MeOH (250 and 550 cm−1), H2O (1200 and 1400 cm−1), and (CF3)2CHOH (1650 and 1900 cm−1). The shift increases in parallel with the hydrogen bonding donating parameter α of solvent, after Kamlet and Taft: 0.9877 (0.93 in ref 78), 1.17 and 1.96, for MeOH, H2O, and (CF3)2CHOH, respectively. ΔνH is clearly not proportional to α. In particular, α changes little between methanol and water, in contrast to ΔνH. On the contrary, near doubling of α between H2O and hexafluoro-2-propanol has less influence on ΔνH. Obviously, it would be possible to build an alternative acidity scale on the n−π* transition of carbonyl compounds having a high sensitivity with respect to proton donating properties of solvents.

(9)

where ε is the depth of the potential well and σ is the most probable (equilibrium) distance. Evidently, in the equilibrium when r = σ, the attractive to repulsive energy ratio equals −2. The same ratio applies to solvent shift, if σ does not change in the excited state. There is little reason to assume the same σ for different states, although the constancy of interaction (Onsager) radius, usually denoted as a, is routinely accepted in standard solvent shift theories.53−59 Relative displacement of potential minima σ*/σ between the excited and the ground state influences critically the solvent shift and related phenomena, such as inhomogeneous site distribution of energies (band shape and width).41,82,83 The effective two-particle model can be used to assess the effect of shrinking/expansion of solute volume upon excitation, bearing in mind that the closest solvent layer is mainly responsible for the repulsive−dispersive solvent shift. The solvent shift Δν is a difference between the potentials of the ground state and the excited state U − U*, where U* = ε* [(σ*/r)12 − 2(σ*/r)6]. The repulsive and repulsive branches can be grouped as follows:41 Δνrep = (ε*σ *12 − εσ 12)r −12

(10)

Δνattr = −2(ε*σ *6 − εσ 6)r −6

(11)

For the shift of (inhomogeneous) band maximum, corresponding to the most probable packing density around the solute molecule (r = σ) one obtains Δνrep = ε*(σ */σ )12 − ε

(12)

Δνattr = −2[ε*(σ */σ )6 − ε]

(13)

Figure 11 shows the contributions of repulsion (in blue) and attraction energies (in red) to solvent shift as a function of relative displacement of potential minima σ*/σ. The potential

5. CONTROVERSIAL ASPECTS OF SOLVATOCHROMISM 5.1. Separation of Dispersive and Repulsive Shifts. The question whether the nonelectrostatic, intermolecular (-atomic) exchange repulsion and dispersion could be extended to solvent shifts seems relevant. It is sometimes accepted that repulsive shifts are universally hypsochomic and the dispersive ones bathochromic.51 Both types of interactions are considered in recent theoretical papers.79−81 Some time ago we proposed a very straightforward approach to the problem41 that will be expanded here. The model consists of a nonpolar solute particle surrounded with a close layer of apolar solvent molecules at an average distance r.41,82,83 Intermolecular potential energy (U) can be approximated to a sum of attractive and repulsive parts, for example in the form of a Lennard-Jones function:

Figure 11. Repulsive and attractive contributions to solvent shift calculated from the Lennard-Jones pair potential of intermolecular interactions, eqs 12 and 13. In contrast to net solvent shift Δν, its components are extremely sensitive with respect to relative shift of potential minima σ*/σ. The potential well of the excited state was assumed to be 20% deeper than in the ground state (ε*/ε = 1.2); hence, the maximum solvent shift is −0.2. Interestingly, if volume contracts in the excited state (σ*/σ < 1) the shift components change sign at certain σ*/σ. 8606


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transfer π−π* chromophores. The hypsochromic shifts of CH2O, AA, and acetone absorption in highly polar and polarizable liquids, such as dimethyl sulfoxide (410, 320, and 350 cm−1, respectively) are somewhat smaller than these in less polarizable acetonitrile (480, 440, and 450 cm−1) and propylene carbonate (Table 1). Bearing in mind that dispersion and induction were exactly compensated in apolar alkanes, it easy to see that, in the absence (or suppression) of an inductive blue shift, the dispersive red shift would reduce the dipolar blue shift Δνdip in strongly polarizable DMSO. Suppression of inductive shift in polar medium is in qualitative agreement with the recently derived expression for electrostatic solvatochromic shifts, incorporating both dipole− dipole and induction effects (a is the radius of solute cavity):87−89

well is assumed by 20% deeper in the excited state, e.g., ε* = 1.2ε. If the potential minima coincide (σ*/σ = 1), the repulsive and attractive contributions have opposite signs, Δνattr = −2Δνrep, and the solvent shift Δν is −0.2, as expected. A small solute volume change can alter dramatically Δνrep and Δνattr but affects little Δν. At a certain point of moderate shrinking of the excited state, the repulsive interaction does not contribute at all, so that the shift is purely dispersive. Indeed, the intermolecular (equilibrium) distance seems often to be compressed in the excited state, at least for π−π* transitions in aromatic hydrocarbons, as well as tetrapyrrolic and polymethine pigments.82−84 Thus, ignoring repulsion in solvent shift theories53−59 might be justified for larger, delocalized chromophores. Paradoxically, at certain σ*/σ values repulsive red shift and attractive blue shift become possible (Figure 11). As for the small >CO chromophore, the average equilibrium distance between the solvent and the excited solute molecules is unknown. The ratio σ*/σ would depend on a balance between the double bond extension10,49 and dispersive attraction in the excited state. As eq 4 was calibrated with Δα for large, planar polyarenes,50 it may yield only very approximate values for the excited state polarizability for CH2O, AA, and acetone. The partitioning to attractive and repulsive energy will remain an important future task for both experiment and theory of solvatochromism. The chromophore volume change on excitation would be a key to the solution. 5.2. About Separation of Optical and Static Permittivity Dependent Shifts. Partitioning of ε dependent (nonequilibrium) dipole−dipole and n dependent induction interactions is fundamentally incorrect even in terms of Onsager theory.85,86 Inconsistency of standard approach to solvatochromism53−59 has been pointed out by Gerhold and Miller,70 Brady and Carr,71 and Klamt.72 As an experimental check, it would be most instructive to compare the ϕ(n2) dependence in both apolar and highly polar solvent sets where the dipole−dipole interaction is absent or, alternatively, reaches a constant limiting value.42,74 The slope of the net ϕ(n2) dependence for push−pull chromophores in solvents with ε > 20 is only slightly less steep than that in n-alkanes (for example, −104 and −8500 cm−1 for 4-nitroanisole, respectively).42 Here about a half of bathochromism is due to induction, and the other half is of dispersive origin, eqs 2 and 3.50 Hence, the strong variability of inductive contribution to solvent “polarity” π* scale as a function of ε, as claimed by Brady and Carr,71 was not confirmed in our work.42 (The π* scale is basing mainly on CT absorption in nitroaromatic compounds).78 The reason is that perfluoroalkanes and benzene are anomalous62,64 and have falsely been included to the apolar set in ref 71. As emphasized already by these authors,71 the induction energy depends on the solute dipole electric field integrated over the surrounding volume. The field strength must diminish dramatically in polar liquids, by a factor of 1/ε. The lack of comparable suppression of induction shift in polar media means that solely the closest layer of solvent molecules is instantly polarized by the solute. Thus, induction behaves similarly to the short-range repulsive−dispersive interaction (see previous section). However, there is some indication about possible suppression of an inductive blue shift for the n−π* transition in carbonyls. Notice that the inductive and dispersive shift components have opposite signs in case of the n−π* transition, whereas both are bathochromic in the above-mentioned charge

Δνind,dip ∼ 9(μg − μe cos ϕ)2 (ε − n2)2 /(2n2 + 1)2 (2ε + 1)(ε − 1)a3 + (μg 2 − μe 2 )(ε − 1)/(2ε + 1)a3

(14)

The formula has been derived within the approximation of solute molecule as a small (point) dipole placed in a spherical cavity embedded in dielectric continuum.87−89 For apolar media (ε = n2) only the second member survives that almost coincides with the expression for induction shift, eq 2. The first member is always positive (or zero) and at large ε (ε ≫ n2) depends on refractive index as (2n2 + 1)−2, whereas the second member remains constant. For example, when n varies between 1.3 and 1.5, this function value diminishes by 37%. This is in qualitative accordance with a slight decrease of positive shift in DMSO with respect to less polarizable CH3CN. In general, eq 14 lacks the repulsive−dispersive shift component and cannot account for the total solvent shift. A remarkable apparent similarity exists between the carbonyl compounds and zwitterionic Dimroth−Reichardt dyes,90 in spite of very different nature of optical transition and size of chromophores. For the CT π−π* transition in solvatochromic Betaine 30 the net positive shift also diminishes with increasing n in highly polar solvents.74 Previously, we have ascribed the effect to a solvent induced change of ionicity and bond alternation in chromophore, caused by the strong reaction field in very polar environment.74 Although this explanation was quite well substantiated, eq 14 could provide an alternative rationale.

6. CONCLUSIONS The existing large body of solvatochromic data accumulated for ketones1,4,5,33−37 was in need of revision. The UV spectra of simple aldehydes are reported for the first time in a number of solvents. Attention is drawn to spurious shifts occurring as a result of variable broadening in solvents with different polarity. An additional pseudoshift between the gas and condensed phase kept at the same temperature is caused by a change in vibronic coupling strength that is a conspicuous phenomenon by itself. In the condensed phase the lower harmonics of >C O stretching mode gain in intensity. The effect is observed in aldehydes FA and AA, as well as in acetone, and seems to be fairly general. It was tentatively rationalized in terms of suppression of large amplitude >CO vibronic modes in the solvent cage. The solvent polarizability dependent shift of n−π* transition is absent in liquid alkanes. Real solvatochromic shifts that are free from artifacts were determined relative to n-alkanes. It is concluded that the dispersive−repulsive and induction shift 8607


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(5) Catalán, J.; Catalán, J. P. On the Solvatochromism of the n ↔ π* Electronic Transitions in Ketones. Phys. Chem. Chem. Phys. 2011, 13, 4072−4082. (6) Bennett, G. E.; Johnston, K. P. UV-Visible Absorbance Spectroscopy of Organic Probes in Supercritical Water. J. Phys. Chem. 1994, 98, 441−447. (7) Ma, H.; Ma, Y. Solvatochromic Shifts of Polar and Non-Polar Molecules in Ambient and Supercritical Water: A Sequential Quantum Mechanics/Molecular Mechanics Study Including Solute-Solvent Electron Exchange-Correlation. J. Chem. Phys. 2012, 137, 214504. (8) Lutz, M. Resonance Raman Studies of Photosynthesis. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1984; Vol. 11, Chapter 5, pp 211−300. (9) Cotton, T. M.; Van Duyne, R. P. Characterization of Bacteriochlorophyll Interactions in Vitro by Resonance Raman Spectroscopy. J. Am. Chem. Soc. 1981, 103, 6020−6026. (10) Moule, D. C.; Walsh, A. D. Ultraviolet Spectra and Excited States of Formaldehyde. Chem. Rev. 1975, 75, 67−84. (11) Blair, J. T.; Krogh-Jespersen, K.; Levy, R. M. Solvent Effects on Optical Absorption Spectra: The 1A1 → 1A2 Transition of Formaldehyde in Water. J. Am. Chem. Soc. 1989, 111, 6948−6956. (12) Sánchez, M. L.; Aguilar, M. A.; del Valle, F. J. O. Solvent Effects on Optical Emission and Absorption Spectra: Theoretical Calculation of the 1(n,π*) Transition of Formaldehyde in Solution. J. Phys. Chem. 1995, 99, 15758−15764. (13) Bader, J. S.; Berne, B. J. Solvation Energies and Electronic Spectra in Polar, Polarizable Media: Simulation Tests of Dielectric Continuum Theory. J. Chem. Phys. 1996, 104, 1293−1308. (14) Naka, K.; Morita, A.; Kato, S. Effect of Solvent Fluctuation on the Electronic Transitions of Formaldehyde in Aqueous Solution. J. Chem. Phys. 1999, 110, 3484−3492. (15) Coutinho, K.; Canuto, S. Solvent Effects in Emission Spectroscopy: A Monte Carlo Quantum Mechanics Study of the n←π* Shift of Formaldehyde in Water. J. Chem. Phys. 2000, 113, 9132−9139. (16) Kawashima, Y.; Dupuis, M.; Hirao, K. Monte Carlo Microsolvation Simulations for Excited States Using a Mixed-Hamiltonian Model with Polarizable and Vibrating Waters: Applications to the Blueshift of the H2CO 1(π*←n) Excitation. J. Chem. Phys. 2002, 117, 248−257. (17) Kongsted, J.; Osted, A.; Pedersen, T. B.; Mikkelsen, K. V.; Christiansen, O. The n → π* Electronic Transition in Microsolvated Formaldehyde. A Coupled Cluster and Combined Coupled Cluster/ Molecular Mechanics Study. J. Phys. Chem. A 2004, 108, 8624−8632. (18) Ö hrn, A.; Karlström, G. A Theoretical Study of the Solvent Shift to the n → π* Transition in Formaldehyde with an Effective Discrete Quantum Chemical Solvent Model Including Non-Electrostatic Perturbation. Mol. Phys. 2006, 104, 3087−3099. (19) DeBolt, S. E.; Kollman, P. A. A Theoretical Examination of Solvatochromism and Solute-Solvent Structuring in Simple Alkyl Carbonyl Compounds. Simulations Using Statistical Mechanical Free Energy Perturbation Methods. J. Am. Chem. Soc. 1990, 112, 7515− 7524. (20) Ten-no, S.; Hirata, F.; Kato, S. Reference Interaction Site Model Self-Consistent Field Study for Solvation Effect on Carbonyl Compounds in Aqueous Solution. J. Chem. Phys. 1994, 100, 7443− 7453. (21) Thompson, M. A. QM/MMpol: A Consistent Model for Solute/Solvent Polarization. Application to the Aqueous Solvation and Spectroscopy of Formaldehyde, Acetaldehyde, and Acetone. J. Phys. Chem. 1996, 100, 14492−14507. (22) Martín, M. E.; Sánchez, M. L.; del Valle, F. J. O.; Aguilar, M. A. A Multiconfiguration Self-Consistent Field/Molecular Dynamics Study of the (n→π*)1 Transition of Carbonyl Compounds in Liquid Water. J. Chem. Phys. 2000, 113, 6308−6315. (23) Kongsted, J.; Mennucci, B.; Coutinho, K.; Canuto, S. Solvent Effects on the Electronic Absorption Spectrum of Camphor Using Continuum, Discrete or Explicit Approaches. Chem. Phys. Lett. 2010, 484, 185−191.

components possess opposite signs and completely cancel each other, at least in less polar solvents. The high sensitivity of the n−π* transition with respect to multipolar and bond dipole moments is observed in nominally nonpolar liquids, such as dioxane, benzene, and CCl4. A set of monofunctional, well behaving (in the sense of smooth ε dependence) polar solvents was applied for characterization of hypsochromism caused by dipole−dipole interaction. The shifts in the dipolar reaction field are very similar for CH2O, AA, and acetone, with an average maximum value reaching 400 cm−1 in highly polar aprotic liquids. The blue shift of AA and acetone absorption grows dramatically with increasing acidity in methanol < H2O < hexafluoro-2-propanol. Pronounced hypsochromism of ketones in protic media has been associated for a long time with stabilization of oxygen lone electron pairs by hydrogen bonds.1,2 Formaldehyde is not stable in water and alcohols.32,34 The excited state dipole moments μe of AA and acetone, as well as the polarizability αe of FA, AA, and acetone, have been estimated. The solvatochromic behavior of the carbonyl chromophore is apparently similar to that of the CT transition in zwitterionic phenol−pyridinium betaines90 in the sense of mutual cancellation of dispersive and induction shifts, high sensitivity with respect to multipolar and hydrogen bonding effects, etc. As to the definition of interaction types, our approach was deliberately traditional and simplistic. Separability of repulsive and dispersive terms, as well as optical and static permittivity components of electrostatic interactions (induction and dipole−dipole), was rationalized. Formaldehyde and acetone have served as popular model chromophores in computer chemical calculations of solvent shifts.11−30,75,79−81 Specific spectral shift due to hydrogen bonding in aqueous environment has been obtained with reasonable accuracy.20−22 The experimental data reported here can serve as a reference to more subtle shifts in aprotic environments, due to dispersive, repulsive, induction, and multipolar interactions.24,29 The computer output of QM/ MM/MD, however closely fitting the facts, cannot be the final destination. Insight must ultimately be provided of how and why the modeling has succeeded or failed.

■

AUTHOR INFORMATION

Corresponding Author

*I. Renge. Phone: +3727-374664. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This paper was supported, in part, by the Estonian Science Agency grants nos. IUT34-27 and IUT2-27. REFERENCES

(1) Balasubramanian, A.; Rao, C. N. R. Evaluation of Solute-Solvent Interactions from Solvent Blue Shifts of n → π* Transitions of CO, CS, NO2 and N=N Groups: Hydrogen Bond Energies of Various Donor-Acceptor Systems. Spectrochim. Acta 1962, 18, 1337−1352. (2) Kashid, S. M.; Bagchi, S. Experimental Determination of the Electrostatic Nature of Carbonyl Hydrogen-Bonding Interactions Using IR-NMR Correlations. J. Phys. Chem. Lett. 2014, 5, 3211−3215. (3) Kolling, O. W. Reaction Field Models for the Solvent Influence upon the Fundamental Carbonyl Vibrational Peak of 2-Butanone in Aprotic Media. J. Phys. Chem. 1996, 100, 16087−16091. (4) Renge, I. Solvent Dependence on n-π* Absorption in Acetone. J. Phys. Chem. A 2009, 113, 10678−10686. 8608


Article

The Journal of Physical Chemistry A (24) Grozema, F. C.; van Duijnen, P. Th. Solvent Effects on the π* ← n Transition of Acetone in Various Solvents: Direct Reaction Field Calculations. J. Phys. Chem. A 1998, 102, 7984−7989. (25) Aidas, K.; Kongsted, J.; Osted, A.; Mikkelsen, K. V.; Christiansen, O. Coupled Cluster Calculation of the n → π* Electronic Transition of Acetone in Aqueous Solution. J. Phys. Chem. A 2005, 109, 8001−8010. (26) Crescenzi, O.; Pavone, M.; De Angelis, F.; Barone, V. Solvent Effects on the UV (n → π*) and NMR (13C and 17O) Spectra of Acetone in Aqueous Solution. An Integrated Car-Parrinello and DFT/ PCM Approach. J. Phys. Chem. B 2005, 109, 445−453. (27) Yoo, S.; Zahariev, F.; Sok, S.; Gordon, M. S. Solvent Effects on Optical Properties of Molecules: A Combined Time-Dependent Density Functional Theory/Effective Fragment Potential Approach. J. Chem. Phys. 2008, 129, 144112. (28) Olsen, J. M.; Aidas, K.; Kongsted, J. Excited States in Solution through Polarizable Embedding. J. Chem. Theory Comput. 2010, 6, 3721−3734. (29) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Sorting Out the Relative Contributions of Electrostatic Polarization, Dispersion, and Hydrogen Bonding to Solvatochromic Shifts on Vertical Electronic Excitation Energies. J. Chem. Theory Comput. 2010, 6, 2829−2844. (30) Aidas, K.; Mikkelsen, K. V.; Mennucci, B.; Kongsted, J. Fluorescence and Phosphorescence of Acetone in Neat Liquid and Aqueous Solution Studied by QM/MM and PCM Approaches. Int. J. Quantum Chem. 2011, 111, 1511−1520. (31) Smith, J. J.; Meyer, B. Fluorescence and Induced Phosphorescence of Formaldehyde in Solid Low-Temperature Solutions. J. Chem. Phys. 1969, 50, 456−459. (32) Bercovici, T.; King, J.; Becker, R. S. Formaldehyde: Comprehensive Spectral Investigation as a Function of Solvent and Temperature. J. Chem. Phys. 1972, 56, 3956−3963. (33) Rao, C. N. R.; Goldman, G. K.; Balasubramanian, A. Structural ́ ← ́ A) Transitions in Aliphatic and Solvent Effects on the n → π* (U Carbonyl Derivatives: Evidence for Hyperconjugation in the Electronically Excited States of Molecules. Can. J. Chem. 1960, 38, 2508−2513. (34) Xu, H.; Wentworth, P. J.; Howell, N. W.; Joens, J. A. Temperature Dependent Near-UV Molar Absorptivities of Aliphatic Aldehydes and Ketones in Aqueous Solution. Spectrochim. Acta, Part A 1993, 49, 1171−1178. (35) Hayes, W. P.; Timmons, C. J. Solvent and Substituent Effects on the n → π* Absorption Bands of Some Ketones. Spectrochim. Acta 1965, 21, 529−541. (36) Dubois, J. E.; Goetz, E.; Bienvenüe, A. Influence des Solvants sur le Transition n → π* des Cétones Saturées. Spectrochim. Acta 1964, 20, 1815−1828. (37) Bayliss, N. S.; Wills-Johnson, G. Solvent Effects on the Intensities of the Weak Ultraviolet Spectra of Ketones and Nitroparaffins − I. Spectrochim. Acta, Part A 1968, 24, 551−561. (38) Freeman, D. E.; Klemperer, W. Electric Dipole Moment of the 1 A2 Electronic State of Formaldehyde. J. Chem. Phys. 1966, 45, 52−57. (39) Vaccaro, P. H.; Zabludoff, A.; Carrera-Patiňo, M. E.; Kinsey, J. L.; Field, R. W. High Precision Dipole Moments in Ã 1A2 Formaldehyde Determined Via Stark Quantum Beat Spectroscopy. J. Chem. Phys. 1989, 90, 4150−4167. (40) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 89th ed.; CRC Press: Boca Raton, FL, 2008. (41) Renge, I. Lennard-Jones Model of Frequency-Selective Barochromism and Thermochromism of Spectral Holes in Glasses. J. Phys. Chem. B 2004, 108, 10596−10606. (42) Renge, I. Excited-State Polarizability and Gas Phase Frequency of the Solvatochromic Polarity Indicator ET(30). Chem. Phys. Lett. 2008, 459, 124−128. (43) Michalik, A. In Wilson and Wilson’s Comprehensive Analytical Chemistry; Svehla, G., Ed.; Elsevier: Amsterdam, 1989; Vol. XIX, pp 99−228. (44) Wohlfarth, Ch. Statische Dielektrizitätskonstanten reiner Flüssigkeiten und binärer flüssiger Mischungen. In Landolt-Börnstein; Neue Serie, Gruppe IV, Band 6; Springer: Berlin, 1991.

(45) Abboud, J.-L. M.; Notario, R. Critical Compilation of Scales of Solvent Parameters. Part I. Pure, Non-Hydrogen Bond Donor Solvents. Pure Appl. Chem. 1999, 71, 645−718. (46) Liao, D. W.; Mebel, A. M.; Hayashi, M.; Shiu, Y. J.; Chen, Y. T.; Lin, S. H. Ab initio Study of the n-π* Electronic Transition in Acetone: Symmetry-Forbidden Vibronic Spectra. J. Chem. Phys. 1999, 111, 205−215. (47) Noble, M.; Lee, E. K. C. The Jet-Cooled Electronic Spectrum of Acetaldehyde and Deuterated Derivatives at Rotational Resolution. J. Chem. Phys. 1984, 81, 1632−1642. (48) Baba, M.; Hanazaki, I.; Nagashima, U. The S1(n,π*) States of Acetaldehyde and Acetone in Supersonic Nozzle Beam: Methyl Internal Rotation and CO Out-of-Plain Wagging. J. Chem. Phys. 1985, 82, 3938−3947. (49) Price, J. M.; Mack, J. A.; Helden, G. v.; Yang, X.; Wodtke, A. M. The Structure of Acetaldehyde in Its First Excited State: Experimental and Theoretical Investigations. J. Phys. Chem. 1994, 98, 1791−1795. (50) Renge, I. On the Determination of Molecular Polarizability Changes upon Electronic Excitation from the Solvent Shifts of Absorption Band Maxima. Chem. Phys. 1992, 167, 173−184. (51) Renge, I. Determination of Electronic Transition Energies for Bare Molecules from Solvent-Shift Measurements. J. Photochem. Photobiol., A 1992, 69, 135−141. (52) Renge, I.; Wild, U. P. Influence of Intermolecular Interactions on the Optical Spectra of Aromatic Impurity Molecules in Solvent Glasses at 8 K. J. Lumin. 1995, 66&67, 305−309. (53) Bakshiev, N. G.; Girin, O. P.; Piterskaya, I. V. Universal Intermolecular Interactions and Their Influence on the Position of Electronic Spectra of Molecules in Binary Solutions. XIV. Dispersion Forces and the Change of Polarizability of Molecules on Optical Excitation. Opt. Spektrosk. 1968, 24, 901−909; Opt. Spectrosc. 1968, 24, 483. (54) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970. (55) McRae, E. G. Theory of Solvent Effects on Molecular Electronic Spectra. Frequency Shifts. J. Phys. Chem. 1957, 61, 562−572. (56) Kawski, A. On the Estimation of Excited-State Dipole Moments from Solvatochromic Shifts of Absorption and Fluorescence Spectra. Z. Naturforsch., A: Phys. Sci. 2002, 57, 255−262. (57) Basu, S. Theory of Solvent Effects on Molecular Electronic Spectra. Adv. Quantum Chem. 1964, 1, 145−169. (58) Amos, A. T.; Burrows, B. L. Solvent-Shift Effects on Electronic Spectra and Excited-State Dipole Moments and Polarizabilities. Adv. Quantum Chem. 1973, 7, 289−313. (59) Suppan, P. Solvatochromic Shifts: the Influence of the Medium on the Energy of Electronic States. J. Photochem. Photobiol., A 1990, 50, 293−330. (60) Maciejewski, A. The Application of Perfluoroalkanes as Solvents in Spectral, Photophysical and Photochemical Studies. J. Photochem. Photobiol., A 1990, 51, 87−131. (61) Reynolds, L.; Gardecki, J. A.; Frankland, S. J. V.; Horng, M. L.; Maroncelli, M. Dipole Solvation in Nondipolar Solvents: Experimental Studies of Reorganization Energies and Solvation Dynamics. J. Phys. Chem. 1996, 100, 10337−10354. (62) Renge, I. Solvatochromic Shift Mechanisms of Centrosymmetric Chromophores in Polar Liquids. J. Photochem. Photobiol., A 2012, 239, 7−16. (63) Khajehpour, M.; Kauffman, J. F. Charge Transfer Kinetics and Solvatochromism of 1-(9-Anthryl)-3-(4-dimethylaniline)Propane in 1,4-Dioxane: Nonideal Quadrupolar Charge Distribution and the Origin of the Dioxane Anomaly. J. Phys. Chem. A 2001, 105, 10316− 10321. (64) Renge, I.; Sild, E. Absorption Shifts in Carotenoids - Influence of Index of Refraction and Submolecular Electric Fields. J. Photochem. Photobiol., A 2011, 218, 156−161. (65) Vogel, A. I. Physical Properties and Chemical Constitution. Part XXIII. Miscellaneous Compounds. Investigation of the So-called Coordinate or Dative Link in Esters of Oxy-acids and in Nitro-paraffins 8609


Article


Form of Solvent Reorganization Energy. Phys. Chem. Chem. Phys. 2012, 14, 13284−13291. (89) Li, X.-Y.; Zhu, Q.; He, F.-C.; Fu, K.-X. Extension of Classical Thermodynamics to Nonequilibrium Polarization. In Thermodynamics; Mizutani, T., Ed.; InTech Publisher: Shanghai, 2011; Chapter 11, pp 205−232. (90) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319−2358.

by Molecular Refractivity Determinations. Atomic, Structural, and Group Parachors and Refractivities. J. Chem. Soc. 1948, 1833−1855. (66) Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441−451. (67) Abe, T. The Dipole Moment and Polarizability in the n-π* Excited State of Acetone from Spectral Solvent Shifts. Bull. Chem. Soc. Jpn. 1966, 39, 936−939. (68) Pašteka, L. F.; Melicherčík, M.; Neogrády, P.; Urban, M. CASPT2 and CCSD(T) Calculations of Dipole Moments and Polarizabilities of Acetone in Excited States. Mol. Phys. 2012, 110, 2219−2237. (69) Hadad, C. M.; Foresman, J. B.; Wiberg, K. B. Excited States of Carbonyl Compounds. 1. Formaldehyde and Acetaldehyde. J. Phys. Chem. 1993, 97, 4293−4312. (70) Gerhold, G. A.; Miller, E. Temperature Dependence of the Solvent Stark Effect. J. Phys. Chem. 1968, 72, 2737−2741. (71) Brady, J. E.; Carr, P. W. An Analysis of Dielectric Models of Solvatochromism. J. Phys. Chem. 1985, 89, 5759−5766. (72) Klamt, A. Calculation of UV/Vis Spectra in Solution. J. Phys. Chem. 1996, 100, 3349−3353. (73) Renge, I.; Mauring, K. Spectral Shift Mechanisms of Chlorophylls in Liquids and Proteins. Spectrochim. Acta, Part A 2013, 102, 301−313. (74) Renge, I. Nonlinear Polarization of Solvatochromic Betaine 30. J. Phys. Chem. A 2010, 114, 6250−6254. (75) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Electronic Absorption Spectra and Solvatochromic Shifts by the Vertical Excitation Model: Solvated Clusters and Molecular Dynamics Sampling. J. Phys. Chem. B 2015, 119, 958−967. (76) Koppel, I. A.; Palm, V. A. The Influence of the Solvent on Organic Reactivity. In Advances in Linear Free Energy Relationships; Chapman, N. B., Shorter, J., Eds.; Plenum: London & New York, 1972; pp 203−280. (77) Marcus, Y. The Properties of Organic Liquids That Are Relevant to Their Use as Solvating Solvents. Chem. Soc. Rev. 1993, 22, 409−416. (78) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, π*, α, and β, and Some Methods for Simplifying the Generalized Solvatochromic Equation. J. Org. Chem. 1983, 48, 2877−2887. (79) Weijo, V.; Mennucci, B.; Frediani, L. Toward a General Formulation of Dispersion Effects for Solvation Continuum Models. J. Chem. Theory Comput. 2010, 6, 3358−3364. (80) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Uniform Treatment of Solute-Solvent Dispersion in the Ground and Excited Electronic States of the Solute Based on a Solvation Model with StateSpecific Polarizability. J. Chem. Theory Comput. 2013, 9, 3649−3659. (81) Cupellini, L.; Amovilli, C.; Mennucci, B. Electronic Excitations in Nonpolar Solvents: Can the Polarizable Continuum Model Accurately Reproduce Solvent Effects? J. Phys. Chem. B 2015, 119, 8984−8991. (82) Renge, I. A Didactic Model of Solvent Shifts, Inhomogeneous Broadening, and Linear and Quadratic Electron-Phonon Coupling. J. Lumin. 2007, 127, 1−6. (83) Renge, I.; Rätsep, M.; Freiberg, A. Intermolecular RepulsiveDispersive Potentials Explain Properties of Impurity Spectra in Soft Solids. J. Lumin. 2011, 131, 262−265. (84) Renge, I. Influence of High Pressure on Optical Impurity Spectra. J. Chem. Phys. 2007, 127, 034504. (85) Onsager, L. Electric Moments of Molecules in Liquids. J. Am. Chem. Soc. 1936, 58, 1486−1493. (86) Böttcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973; Vol. I. (87) Li, Y.-K.; Zhu, Q.; Li, X.-Y.; Fu, K.-X.; Wang, X.-J.; Cheng, X.-M. Spectral Shift of the n → π* Transition for Acetone and Formic Acid with an Explicit Solvent Model. J. Phys. Chem. A 2011, 115, 232−243. (88) Ren, H.-S.; Li, Y.-K.; Zhu, Q.; Zhu, J.; Li, X.-Y. Spectral Shifts of the n → π* and π → π* Transitions of Uracil Based on a Modified 8610


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