4010
J. Phys. Chem. B 2001, 105, 4010-4017
Perturbations of the Fully Resolved Electronic Spectra of Large Molecules by the Internal Rotation of Attached Methyl Groups. Influence of Complex Formation† Timothy M. Korter and David W. Pratt* Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: December 11, 2000; In Final Form: February 23, 2001
Rotationally resolved S1rS0 fluorescence excitation spectra of three methylindoles and their single atom van der Waals complexes with argon have been obtained. Each spectrum is extensively perturbed by the hindered internal rotation of the methyl group. Analyses of these perturbations show that the barriers to such motion are substantially increased by complex formation. Barriers of this type are primarily electronic in origin. Thus, even at the relatively large distances (3.5 Å) found in van der Waals complexes, the attachment of a weakly bound argon atom has a significant effect on the electron distribution in the indole ring.
Introduction The field of electronic spectroscopy, like that of surface science, has advanced immensely in the last few years. The recording of the near-UV spectra of naphthalene and perdeuterated naphthalene with fully resolved rotational structure1 can be considered as marking the beginning of the modern era in this field. This era is characterized by studies of such spectra at eigenstate resolution, exposing for the first time all of the underlying quantum states in the system and the interactions that are responsible for them, using molecular beam machines, high resolution lasers, and state-of-the-art computer software. Subsequent to its discovery, the technique has been widely employed for the determination of the structures of many large molecules and their complexes in the gas phase, in different electronic states, and for exploring their dynamical behavior following the absorption of light.2 We focus in this report on the special sensitivity of the fully resolved electronic spectrum of a large molecule to the torsional motion of an attached methyl group, and on the changes in the potential energy surfaces that govern such motions that are produced when a rare gas atom is attached to the molecule by a weak van der Waals “bond”. Rotationally resolved studies are sensitive to torsional motions because of their intrinsic angular momenta, which couple to overall molecular rotation and produce detectable splittings and/or perturbations in the spectra.3 Here, we exploit this sensitivity in a study of a series of methyl-substituted indoles and their single Ar atom complexes, in their S0 and S1 electronic states. Somewhat surprisingly, we find that the methyl groups attached to the bare molecules and their complexes exhibit different barriers to internal rotation, despite the fact that the Ar atom is located at some distance (∼3.5 Å) away from the aromatic plane. Apart from the methyl group, and the weakly bound Ar atom, the molecules investigated here all are planar molecules, encouraging comparison with analogous studies in surface science. A number of groups have used vibrational spectroscopy to study the rotation of physisorbed and chemisorbed species on well-defined structures. Additionally, adsorbates such as NH3 † Part of the special issue “John T. Yates, Jr. Festschrift”. We dedicate this paper to our friend and colleague, John T. Yates, Jr., on the occasion of his 65th birthday.
and PF3 have been extensively studied by ESDIAD, a method that images chemical bond directions.4 An interesting question is whether such motions are influenced by the surfaces to which they are attached. There also is a need to understand the coupling between adsorbate vibrations and the surface electrons, which may influence the rates by which such vibrations relax.5 Thus, there are significant conceptual links between the fields of electronic spectroscopy and surface science, especially as practiced at the University of Pittsburgh. Experimental Section 1-Methylindole (97 + %), 3-methylindole (98%), and 5-methylindole (99%) were purchased from Aldrich and used as received. Dry argon gas (99.999%) was used for the van der Waals complex experiments. High resolution data were obtained using the molecular beam laser spectrometer described in detail elsewhere.6 The molecular beam was formed by the expansion of the molecule of interest (typically heated to ∼350 K) in an Ar carrier gas (∼500 Torr) through a 240 µm quartz nozzle into a differentially pumped vacuum system. Van der Waals complexes were formed by expansions of the bare molecule in ∼100 Torr of Ar. The expansion was skimmed 2 cm downstream of the nozzle with a 1 mm skimmer and crossed 13 cm further downstream by a CW ring dye laser operating with R590 and intracavity frequency doubled in BBO, yielding ∼300 µW of ultraviolet radiation. Fluorescence was collected using spatially selective optics, detected by a photomultiplier tube and photon counting system, and processed by a computerized data acquisition system. Relative frequency calibrations of the excitation spectra were performed with a near-confocal interferometer having a mode-matched FSR of 299.7520 ( 0.0005 MHz at the fundamental frequency of the dye laser. Absolute transition frequencies were determined by comparison to transition frequencies in the iodine absorption spectrum and are accurate to ( 30 MHz.7 Results and Interpretation A. 1-Methylindole (1MI). Figure 1 shows the rotationally resolved fluorescence excitation spectrum of the 000 band of 1MI. Though not apparent at first glance, a close examination
10.1021/jp004451h CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001
Internal Rotation of Attached Methyl Groups
J. Phys. Chem. B, Vol. 105, No. 18, 2001 4011 TABLE 1: Inertial Parameters of 1-Methylindole in Its Ground (S0) and Excited (S1) Electronic States Derived from an Analysis of Its Rotationally Resolved S1 r S0 Fluorescence Excitation Spectruma A-lines
S0
S1
Figure 1. Rotationally resolved fluorescence excitation spectrum of the origin band of the S1 r S0 transition of 1-methylindole. The origin band is the superposition of two subbands which are separated by 0.752 cm-1. The top trace is the experimental spectrum. The second and third traces are the calculated 0A′ r 0A′′ and 0E′ r 0E′′ subbands, respectively.
of this spectrum reveals that it consists of two subbands, which we call the 0A′ r 0A′′ and 0E′ r 0E′′ sub-torsional bands or, more simply, the A and E subbands. Each of these subbands is a b-type spectrum, as it lacks the distinguishing Q-branch features of a- and c-type spectra. The S1 r S0 transition moment (TM) lies in the plane of the molecule, close to the b inertial axis. The splitting of the origin band of 1MI into two subbands is easily understood.8 The torsional motion of the methyl group in 1MI is opposed by a 3-fold barrier, V3 (φ), in both electronic states. For finite V3, the nominally degenerate levels lying below the barrier interact, yielding a lower nondegenerate level A and an upper doubly degenerate level E. Allowed transitions in an electronic spectrum connect the A(E) levels in the S0 state with the A(E) levels in the S1 state. The energy difference between the A and E levels depends on the magnitude of V3 which, in general, is different in different electronic states. Thus, all vibronic bands of a molecule containing a hindered methyl group typically are split into two subbands, whose frequency difference is related to the sum (or difference) of the tunneling splittings in the two states. This difference, which we denote as ∆νE-A, is 0.752 cm-1 in 1MI, as shown in Figure 1. We are interested in extracting from these data the values of V3 in both electronic states. This requires a frequency and intensity fit of the entire spectrum. Details of our fitting procedures have been described previously.6 Briefly, for all bare molecules examined here, the rotational constants of the S0 state were estimated by ab initio methods, using MP2/6-31G**,9 the rotational constants of the S1 state were estimated using the previously determined values for indole,10 and both sets of constants were refined by interactive least-squares fits of the fully resolved spectra. The corresponding parameters of the Ar atom complexes of the methylindoles were initially estimated using the previously determined values for indole-Ar.11 All rotational analyses were performed with the computer program IAR12 that utilizes Watson’s Hamiltonian13 for the distortable asymmetric rotor and the internal axis method14 to describe the motion of the methyl group. The same Hamiltonian was applied
A′′ B′′ C′′ Da′′ Db′′ Dc′′ k′′ ∆I′′
2651.4(1) 1305.3(1) 879.8(1)
A′ B′ C′ Da′ Db′ D c′ k′ ∆I′
2586.7(1) 1301.2(1) 869.8(1)
ν0 assign OMC
-0.52 -3.35
-0.50 -2.74 34 537.635(1) 235 1.55
E-lines 2651.0(1) 1305.2(1) 879.8(1) 20.05(10) 12.95(6.76) 0.00 -0.52 -3.43 2581.0(1) 1299.3(1) 869.9(1) 311.02(9) 197.42(50) 0.00 -0.50 -3.77 34 538.387(1) 292 2.41
a A, B, and C are the rotational constants (MHz); D , D , and D are a b c the torsion-rotation coupling terms (MHz); κ is Ray’s asymmetry 2 parameter; ∆I is the inertial defect (u ‚ Å ); ν0 is the band origin (cm-1); and OMC is the standard deviation of the fit (MHz).
in the fit of each spectrum. However, the fits of the bare molecule spectra typically yielded centrifugal distortion terms whose values were less than their standard deviations. Hence, these terms were not included in the final fits. Our software package includes torsion-rotation perturbation terms to account for the coupling of internal rotation with overall rotation15 as well as Eulerian angles to treat the phenomenon of inertial axistilting.16 Independent fits were made of the two subbands in each spectrum. In the case of 1MI, rigid-rotor fits of the line positions in the spectrum shown in Figure 1 yielded 235 assignments in the A subband and 292 assignments in the E subband, with OMC values of 1.55 and 2.41 MHz, respectively. Both standard deviations are significantly less than the single rovibronic line widths of 20 MHz. Fits of the line intensities showed that each subband is an ab hybrid band with 98% b-type and 2% a-type character. The rotational temperature of the spectrum in Figure 1 is Tr ≈ 2K. At higher temperatures, where higher J states are populated, an interesting effect is observed; transitions terminating in excited state levels with J > 10 are shifted from their predicted positions by as much as 100 MHz. Inclusion of centrifugal distortion terms in the Hamiltonians of the two states did not improve this situation. It is possible that these perturbations are responsible for the rotationally mediated dynamics of 1MI reported by Leach, et al.17 In any event, the affected lines have been excluded from the fit reported here. Table 1 lists the spectroscopic parameters determined from analyses of the data in Figure 1. All inertial constants are known to a precision of 0.1 MHz, making possible reliable structural and dynamical conclusions. For example, we see that the A, B, and C constants of the A and E levels of the S1 state are substantially different from the corresponding constants of the A and E levels of the S0 state. This shows that the two electronic states of 1MI have substantially different geometries. Comparable changes in A, B, and C occur on excitation of indole to its S1 state.10 Thus, the attachment of a methyl group to the nitrogen atom of the indole ring does not appear to have a major affect on its electronic structure in either state.
4012 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Figure 2. Rotationally resolved fluorescence excitation spectrum of the origin band of the S1 r S0 transition of 1-methylindole-Ar, shifted 16.6 cm-1 to the red of the S1 r S0 origin band of 1-methylindole. The spectrum is the superposition of two subbands which are separated by 0.668 cm-1. The top trace is the experimental spectrum. The second and third traces are the calculated 0A′ r 0A′′ and 0E′ r 0E′′ subbands, respectively.
Also of interest in Table 1 are the parameters that are sensitive to the internal rotation of the methyl group. These include the differences in the A, B, and C constants of the A and E levels, the torsion-rotation parameters Da, Db, and Dc, and the aforementioned subband splitting, ∆νE-A. The torsion-rotation parameters give information about the internal rotation constant F and the axis about which the motion is occurring. From the data for 1MI, we find the (average) value for F in both states is 5.13 ( 0.05 cm-1. The motion is about an in-plane axis that makes a significant angle with both a and b. And all three sets of parameters give information about the barrier to internal rotation. For example, the fact that the differences in the A, B, and C values of the A and E levels are larger in the S1 state than in the S0 state shows immediately that V3(S1) < V3(S0) because these differences have their origin in the differences in the degree of vibrational averaging in each state, along each of the principal axes. Note, also, that Da, Db (S1) > Da, Db(S0), which again is a consequence of the smaller barrier in the S1 state. The values V3(S0) ) 262 ( 20 cm-1 and V3(S1) ) 106 ( 1 cm-1 may be derived from these values.8,15 And, finally, the measured subband splitting of ∆νE-A ) 0.752 cm-1 (with the E subband lying to higher energy) provides an independent check of the sign of the change in V3, whether positive or negative. All of these data show, then, that the methyl torsional barrier of 1MI in its S1 state is less than half that of the S0 state. B. 1-Methylindole-Ar (1MIA). Figure 2 shows the rotationally resolved fluorescence excitation spectrum of the 000 band in the S1 r S0 transition of 1MIA, the single Ar atom complex of 1MI. Comparing this figure to Figure 1, we see that there are several differences in the two spectra. The 000 band of 1MIA is red-shifted by 16.6 cm-1 with respect to that of the bare molecule, owing to differences in the binding energy of the Ar atom to the bare molecule in its two electronic states. The fully resolved spectrum of the 000 band of 1MIA again is split into two subbands, as in the case of 1MI itself. However, the splitting of the two subbands is different, ∆νE-A ) 0.668
Korter and Pratt
Figure 3. Portion of the high-resolution spectrum of 1-methylindoleAr at full experimental resolution, extracted from approximately midway between the two subband origins. The top trace is the experimental spectrum. The second and third traces show the separate calculated contributions of the two subbands in this region.
cm-1 in the complex compared to ∆νE-A ) 0.752 cm-1 in the bare molecule. This shows that the attachment of a weakly bonded Ar atom significantly modifies the restricted internal rotation of the methyl group, in one of the two states, or both. And, finally, a comparison of Figures 1 and 2 also shows that the hybrid band characters of the two subbands in 1MIA are different from those in 1MI because they clearly exhibit strong Q-branch structure. The difference in band types in the two spectra can be traced to a reorientation of the inertial axes upon complex formation. Recall that Ia < Ib < Ic for a polyatomic molecule. In the bare molecule, the largest moment (Ic) is perpendicular to the indole plane. However, attachment of an Ar atom above this plane, roughly along c, dramatically increases the moments about a and b, requiring a “re-labeling” of the axes. Thus, the largely b-type spectrum of the bare molecule is expected to be transformed into a largely c-type spectrum in the complex, as observed. No large reorientation of the electronic TM occurs. In the case of 1MIA, fits of the spectra shown in Figure 2 yielded 273 assignments in the A subband and 336 assignments in the E subband, with OMC values of 2.35 and 2.41 MHz, respectively, again significantly less than the line width (18 MHz). Each subband is a ∼100% c-type band; the derived value of the rotational temperature is Tr ≈ 2 K. Figure 3 shows a comparison of corresponding line positions and intensities in portions of the experimental and computed spectra, to illustrate the quality of the fit. As can be seen, there is no evidence for the perturbations observed in the spectrum of the bare molecule in the spectrum of the complex. Possibly, this is because fewer of the high J levels are populated at the lower Tr. 1MIA is also, however, a significantly less rigid molecule. Inclusion of centrifugal distortion terms in the Hamiltonians used for the fit decreased the OMCs by more than a factor of 2. Table 2 lists the parameters derived from our fit of the spectrum of 1MIA. Examining these results, we see first that 1MIA has substantially smaller rotational constants than 1MI in both electronic states. These changes reflect the larger moments of inertia of the complex, compared to the bare molecule. As discussed in detail elsewhere,11,18 comparison of the two sets of constants using Kraitchman’s equations14 makes
Internal Rotation of Attached Methyl Groups
J. Phys. Chem. B, Vol. 105, No. 18, 2001 4013
TABLE 2: Inertial Parameters of 1-Methylindole-Argon in Its Ground (S0) and Excited (S1) Electronic States Derived from an Analysis of Its Rotationally Resolved S1 r S0 Fluorescence Excitation Spectruma A-lines
E-lines
1056.1(1) 770.4(1) 665.1(1)
1056.5(1) 770.6(1) 665.3(1) 5.69(9) 0.00 5.26(8) 7.6(1.1) -4.0(1.5) 0.8(5) 0.2(1.6) 0.5(3) -0.46 -374.57
S0
A′′ B′′ C′′ Da′′ Db′′ Dc′′ ∆K′′ ∆JK′′ ∆J′′ δK′′ δJ′′ k′′ ∆I′′
S1
A′ B′ C′ Da′ Db′ Dc′ ∆K′ ∆JK′ ∆J′ δK′ δJ′ k′ ∆I′
7.6(1.1) -3.9(1.5) 0.5(4) -0.6(1.3) 1.2(2) -0.32 -384.66
1017.2(1) 790.8(1) 672.5(1) 88.34(9) 48.74(38) 89.77(7) 9.8(1.1) -5.5(1.6) 1.3(5) -0.1(1.4) 0.8(3) -0.31 -384.38
ν0 Assign OMC
34521.096(1) 300 2.14
34521.764(1) 365 2.19
4.8(9) -1.9(1.3) 0.1(4) -1.1(1.3) 1.0(2) -0.46 -374.65 1017.3(1) 790.7(1) 672.7(1)
a A, B, and C are the rotational constants (MHz); D , D , and D are a b c the torsion-rotation coupling terms (MHz); ∆K′′ etc. are the Watson distortion terms (kHz); k is Ray’s asymmetry parameter; ∆I is the inertial defect (u‚Å2); ν0 is the band origin (cm-1); and OMC is the standard deviation of the fit (MHz).
TABLE 3: Coordinates of the Argon Atom in the Principal Axis Frames of 1-Methylindole and 1-Methylindole (Ar)a state
coordinate
unsubstituted frame
substituted frame
S0
|a| |b| |c| |r|
0.8201 ( 0.0004 0.2019 ( 0.0052 3.3758 ( 0.0005 3.4799 ( 0.0004
2.0414 ( 0.0008 1.7046 ( 0.0052 0.1972 ( 0.0049 2.6668 ( 0.0003
S1
|a| |b| |c| |r|
0.6493 ( 0.0005 0.2015 ( 0.0050 3.3652 ( 0.0005 3.4332 ( 0.0004
1.8880 ( 0.0011 1.8211 ( 0.0010 0.2027 ( 0.0050 2.6310 ( 0.0003
a The distance between the argon and 1-methylindole’s center-ofmass is given by "r". Errors in the argon displacements were determined by propagating the errors in the measured rotational constants through the Kraitchman analysis. All values are in Å.
possible the determination of the center-of-mass (COM) coordinates of the attached atom in the complex, in both electronic states. The results of this analysis are listed in Table 3. Only the absolute values of these coordinates can be determined by this method, owing to the displacement-squared dependence of the moments of inertia. But the eight possible positions are reduced to four by the (1MI) plane of symmetry. Of these four, only one corresponds to the true (vibrationally averaged) position of the Ar atom. The preferred position is above the fivemembered ring and tilted toward the nitrogen, by analogy to indole-Ar, where the Ar atom position was unambiguously determined.11 In 1MIA, as in indole-Ar, there is a significant photoinduced change in the position of the attached atom,
Figure 4. Rotationally resolved fluorescence excitation spectrum of the origin band of the S1 r S0 transition of 3-methylindole. The origin band is the superposition of two subbands which are separated by 0.0306 cm-1. The top trace is the experimental spectrum. The second and third traces are the calculated 0A′ r 0A′′ and 0E′ r 0E′′ subbands, respectively.
consistent with the red shift of the origin band of the complex relative to the bare molecule. Perhaps of greater interest is the additional finding that the barrier to internal rotation of the methyl group in 1MIA is different from that in the bare molecule. This is immediately apparent from a comparison of Figures 1 and 2. In the complex, the A and E subband origins are separated by ∆νE-A ) 0.668 cm-1, compared to 0.752 cm-1 for 1MI itself. This clearly indicates that the potential energy surface for methyl torsion is different in either one or both electronic states. The values of V3 derived from the D values in Table 2 are V3(S0) ) 306 ( 2 cm-1 and V3(S1) ) 127 ( 1 cm-1, with F ) 5.60 ( 0.05 cm-1. Thus, both states are affected by complex formation. The ground state barrier increases by 13%, and the excited state barrier increases by 20%, on attachment of the weakly bound Ar atom to the bare molecule. The value of F also increases by ∼10%. And, again, the excited state barrier in 1MIA is substantially less than that of the ground state, as in 1MI itself. C. 3-Methylindole-Ar (3MIA). A similar phenomenon was observed in 3-methylindole (3MI) and its single Ar atom van der Waals complex (3MIA), whose S1 r S0 origin band is red shifted from the corresponding band of 3MI by 31.8 cm-1. The rotationally resolved S1 r S0 fluorescence excitation spectrum of the bare molecule has been described by Meerts and coworkers.19 As shown in Figure 4, it has characteristic, central a-type Q-branches, and is composed of two, superimposed subbands, each of which is an ab-hybrid band with 80% a and 20% b character. In contrast, the rotationally resolved spectrum of the Ar complex, shown in Figure 5, is an abc hybrid band, with ∼40% a, ∼40% b, and ∼20% c character. These are similar to our findings for indole-Ar.11,20 The change in band type on complexation again is due to a relabeling of the inertial axes, owing to the additional mass of the Ar atom. And again, as in the case of 1MI and 1MIA, the spectra of 3MI and 3MIA exhibit subband splittings that are different, as can be seen by comparing Figures 4 and 5.
4014 J. Phys. Chem. B, Vol. 105, No. 18, 2001
Korter and Pratt TABLE 5: Inertial Parameters of 3-Methylindole-Argon in Its Ground (S0) and Excited (S1) Electronic States Derived from an Analysis of Its Rotationally Resolved S1 r S0 Fluorescence Excitation Spectruma A-lines
S0
Figure 5. Rotationally resolved fluorescence excitation spectrum of the origin band of the S1 r S0 transition of 3-methylindole-Ar, shifted 31.8 cm-1 to the red of the S1 r S0 origin band of 3-methylindole. The spectrum is the superposition of two subbands which are separated by 0.0281 cm-1. The top trace is the experimental spectrum. The second and third traces are the calculated 0A′ r 0A′′ and 0E′ r 0E′′ subbands, respectively.
TABLE 4: Inertial Parameters of 3-Methylindole in Its Ground (S0) and Excited (S1) Electronic States Derived from an Analysis of Its Rotationally Resolved S1 r S0 Fluorescence Excitation Spectruma
S0
S1
A-lines
E-lines
A′′ B′′ C′′ Da′′ Db′′ Dc′′ k′′ ∆I′′
2604.1(1) 1269.1(1) 858.0(1) -0.53 -3.29
2604.5(1) 1269.0(1) 858.0(1) 2.32(21) 0.00 0.52(15) -0.53 -3.28
A′ B′ C′ Da′ Db′ Dc′ k′ ∆I′
2566.8(1) 1250.9(1) 845.6(1) -0.53 -3.28
2567.0(1) 1250.8(1) 845.6(1) 14.17(20) 0.00 0.51(15) -0.53 -3.30
34874.687(1) 287 3.56
34874.718(1) 570 3.91
ν0 assign OMC
a A, B, and C are the rotational constants (MHz); D , D , and D are a b c the torsion-rotation coupling terms (MHz); κ is Ray’s asymmetry 2 parameter; ∆I is the inertial defect (u ‚ Å ); ν0 is the band origin (cm-1); and OMC is the standard deviation of the fit (MHz).
Tables 4, 5, and 6 list the parameters that were derived from analyses of these data. The spectrum of the bare 3MI molecule was fit with rigid rotor Hamiltonians for each electronic state, yielding OMC's of 3.56 and 3.91 MHz for the A- and E-lines, respectively. The derived values of the inertial parameters in Table 4 are in excellent agreement with those of Remmers, et al.19 The spectrum of the 3MIA complex was fit with a distortable rotor Hamiltonian for each electronic state, yielding the parameters listed in Table 5. A total of 516 assignments were made in the A band and 661 assignments were made in the E band, yielding OMCs of 2.61 and 2.83 MHz, respectively.
S1
A′′ B′′ C′′ Da′′ Db′′ Dc′′ ∆K′′ ∆JK′′ ∆J′′ δK′′ δJ′′ k′′ ∆I′
925.0(1) 842.8(1) 648.8(1)
8.3(7) -4.9(8) 1.4(2) -1.8(3) 0.52(8) 0.41 -367.04 923.8(1) 840.8(1) 655.6(1)
E-lines 924.9(1) 842.7(1) 648.8(1) 0.13(8) 5.77(2.46) 0.68(8) 7.8(5) -5.0(6) 1.6(1) -1.1(2) 0.60(7) 0.40 -367.16
A′ B′ C′ D a′ Db′ Dc′ ∆K′ ∆JK′ ∆J′ δK′ δ J′ k′ ∆I′
1.4(6) 0.53(64) 0.69(13) -0.035(0.132) 0.21(6) 0.38 -377.27
923.8(1) 840.6(1) 655.6(1) 3.61(7) 7.45(1.71) 3.69(8) 0.32(38) 1.24(47) 0.73(12) 0.84 (17) 0.24(6) 0.38 -377.36
ν0 assign OMC
34842.879(1) 516 2.61
34842.907(1) 661 2.83
a A, B, and C are the rotational constants (MHz); D , D , and D are a b c the torsion-rotation coupling terms (MHz); ∆K′′ etc. are the Watson distortion terms (kHz); κ is Ray’s asymmetry parameter; ∆I is the inertial defect (u‚Å2); ν0 is the band origin (cm-1); and OMC is the standard deviation of the fit (MHz).
TABLE 6: Coordinates of the Argon Atom in the Principal Axis Frames of 3-Methylindole and 3-Methylindole (Ar)a state
coordinate
unsubstituted frame
substituted frame
S0
|a| |b| |c| |r|
0.2385 ( 0.0005 0.5316 ( 0.0026 3.4355 ( 0.0005 3.4845 ( 0.0004
1.6732 ( 0.0031 2.0429 ( 0.0023 0.3971 ( 0.0017 2.6706 ( 0.0003
S1
|a| |b| |c| |r|
0.1931 ( 0.0008 0.4353 ( 0.0028 3.3965 ( 0.0005 3.4297 ( 0.0004
1.1935 ( 0.0045 2.3133 ( 0.0022 0.3642 ( 0.0022 2.6284 ( 0.0003
a The distance between the argon and 1-methylindole’s center-ofmass is given by "r". Errors in the argon displacements were determined by propagating the errors in the measured rotational constants through the Kraitchman analysis. All values are in Å.
The single rovibronic line width is 25 MHz; the rotational temperature was ∼3 K. 3MIA exhibits significantly smaller changes in its rotational constants on S1 r S0 excitation, as is also evidenced by the relatively symmetric appearance of its spectrum. Substitution coordinates of the Ar atom in both electronic states are listed in Table 6. Significant changes in these displacements are again observed on S1 excitation. The A-E origin spacing in 3MIA is 0.0281 cm-1, compared to 0.0306 cm-1 for the bare molecule. The corresponding V3 barriers are; for 3MI, V3(S0) ) 500 ( 40 and V3(S1) ) 301 ( 1 cm-1 (F ) 5.17 cm-1)19, and for 3MIA, V3(S0) ) 522 ( 50 and V3(S1) ) 331 ( 10 cm-1 (F ) 5.60 cm-1, an assumed value based on the results for 1MIA). The attachment of the
Internal Rotation of Attached Methyl Groups
J. Phys. Chem. B, Vol. 105, No. 18, 2001 4015
TABLE 7: Barriers to Internal Rotation of the Methyl Groups in 1-, 3-, and 5-Methylindole and Their Single Atom van der Waals Complexes with Argon, in Their S0 and S1 Electronic States V3(S0), cm-1 molecule
bare
1MI
262 (283)a,d 500 (500)b
306
135b,d (133)a,d
>135
3MI 5MI
complex
522
V3(S1), cm-1 bare
complex
106 (116)a,d 301 (301)b (308)c 85b,d (85)a,d (81)c,d
127 331 >85
a Ref 21. b Ref 19. c Ref 22. d Smaller V terms were included in 6 the previous fits of the spectra of 1MI and 5MI.
weakly bound Ar atom to 3MI again increases the barrier to internal rotation, by 4% in the S0 state and by 10% in the S1 state. We also examined the spectra of 5-methylindole (5MI) and its single argon atom complex (5MIA), to probe the influence of complex formation on the dynamical behavior of a methyl group attached to the six-membered ring. 5MI has previously been studied in high resolution by Remmers, et al.19 Our spectrum of the bare molecule is identical to theirs; it exhibits two subbands separated by 0.88 cm-1. The E subband spectrum is blue shifted with respect to the A subband spectrum, indicating a decrease in V3 on S1 excitation. The E subband also is much less intense. This pattern is repeated in the argon atom complex, shifted by 28.3 cm-1 with respect to that of the bare molecule. Thus, we find that the spectrum of 5MIA again is split into two subbands, with the E subband shifted to higher energy. However, the separation of the two subbands is substantially different, ∼0.78 cm-1, and the E subband intensity is further reduced, compared to that of the A subband. We find ratios of A/E ≈ 4/1 in 5MI and A/E ≈ 6/1 in 5MIA. These ratios may have their origins in quantum interference effects;21 additionally, nonrigid body effects and/or other torsionally mediated dynamics may play a role. Whatever their origin, these effects precluded a definitive assignment of the spectrum of 5MIA. The data do show, however, that attachment of a weakly bound Ar atom affects the methyl rotor barriers in 5MI itself. These have been determined to be V3(S0) ) 135 ( 6 cm-1 and V3(S1) ) 85 ( 1 cm-1 by Remmers, et al.19 Although we are unable to extract the corresponding values for 5MIA from our data, it is likely that they are also increased by complex formation because ∆νE-A is smaller in 5MIA than in 5MI itself. Discussion Summarized in Table 7 are the barriers to internal rotation of the methyl groups in the three methylindoles studied in this work, together with their single atom van der Waals complexes with argon, in both electronic states. The bare molecule values are in substantial agreement with those determined earlier, in some cases by different methods.19,22,23 All authors agree that V3 depends significantly on the positions of attachment of the methyl group to the indole ring, and also depends significantly on the electronic state of the molecule to which the methyl group is attached. Reported here is the new finding that V3 also changes when an argon atom is attached to the indole ring. V3 increases by 17% on complexation of 1MI in its S0 state, and by 4% on complexation of 3MI in its S0 state. These are remarkable results,
especially given the data in Tables 3 and 5 (see also ref 19), which show that the distance between the argon atom and the center of mass of the indole ring is ∼3.5 Å in all three molecules! We are not the first to detect such an effect. The interaction of rare gas atoms with methyl rotors has been extensively studied in ground-state molecules using microwave spectroscopy. Examples include methanol-Ar,24 N-methylpyrrole-Ar,25 pfluorotoluene-Ar,26 and 1,1-difluoroethane-Ar.27 Typically, barrier heights are observed to increase on complexation but there are some interesting exceptions. Thus, the attachment of a single argon atom perpendicular to a ring plane can change the symmetry of the potential function, as in N-methylpyrrole and p-fluorotoluene. And, in other cases, sharp decreases in V3 are observed, as in methanol, owing to couplings with other large amplitude motions.28 The formation of weakly bound complexes also can directly influence motions of this type, such as pseudorotation.29 Such effects are less well studied in electronically excited states. However, Sammeth, et al.23 observed that the torsional barrier height in the S1 state of 5MI increases from 81 to 85 cm-1 on complexation with a single He atom. Similarly, Takayanagi and Hanazaki30 discovered, in analogous low resolution studies of 1:1 complexes of m-tolunitrile (MTN) with Ne, Ar, and Kr, that the V3 values in the S1 state increase monotonically with the size of the rare gas atom, from 38.0 cm-1 in MTN itself to 41.5 cm-1 in MTN-Ne, 48.5 cm-1 in MTN-Ar, and 55.0 cm-1 in MTN-Kr. More generally, complex-induced changes in the potential energy surfaces along different vibrational coordinates have been the subject of a number of investigations. One of the earliest examples was revealed in the S1rS0 fluorescence excitation spectrum of s-tetrazine-Ar,31 a complex in which the Ar atom lies on the out-of-plane C2 axis at a distance of ∼3.4 Å from the ring, in both electronic states. Brumbaugh, et al.31 observed that all complex vibronic bands containing overtones of ν16a or ν16b exhibit much smaller red shifts than do other bands, the two affected modes being out-of-plane modes. Normally, the spectrum of such a complex appears as a “shadow spectrum” of the spectrum of the bare molecule, with all vibronic bands of the complex being shifted by the same (small) amount from the corresponding bands of the bare molecule, demonstrating that complexation has little effect on its electronic and vibrational structure. Other exceptions to this “rule” exist. One of the most vivid exceptions is the demonstration that different isomers of glyoxal-Ar exhibit different decay properties in their S1 state. An Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S1 f T1 ISC transition of glyoxal.32 But most exceptions involve normal modes in complexes that have a component of the atomic displacements along the direction of the van der Waals bond,33 a fact that is relevant to IVR and vibrational predissociation behavior. The especially interesting finding in the present work is, then, that the observed changes in the 3-fold barriers to internal rotation on complexation involve a vibrational mode that has no net atomic displacement along the direction of the van der Waals bond. The torsional motion is “strictly” in-plane, or at least about an axis that lies in the plane of the bare molecule.34 The key to understanding this rather intriguing result lies in the origin of the torsional potentials of methyl rotors attached to unsaturated “linkages” in organic molecules. As hypothesized originally by Hehre, et al.,35 and elaborated upon further by Dorigo, et al.,36 the barriers hindering the internal rotation of a methyl group are dominated by π-electron effects. Thus, there
4016 J. Phys. Chem. B, Vol. 105, No. 18, 2001 is a substantial preference for a methyl group that is attached to an unsaturated linkage to adopt a conformation in which one of its CsH bonds eclipses the multiple bond, because the resulting overlap between the out-of-plane CsH hydrogens and the π linkage is energetically more favorable. To be sure, “steric” effects also can play a role, but only if the affected atoms come to within a van der Waals radius or less of each other. We believe that the methylindoles' sensitivity to the attachment of a rare gas atom perpendicular to the plane derives from the influence of such attachments on the π-electron distributions, which again are perpendicular to the plane. Thus, in this respect, the present results are not surprising at all! The largest barrier for the molecules examined here is for 3MI, V3(S0) ≈ 500 cm-1. We believe that this barrier is largely electronic in origin. The preferred conformation of the methyl group in 3MI according to our MP2/6-31G** calculations9 has a CsH hydrogen eclipsing the C2sC3 bond. This bond clearly has large π-character, in the ground state. Excitation of 3MI to its S1 state reduces the barrier to ∼300 cm-1. No conformational change is observed.19,23 Hence, the reduction of the barrier by ∼40% is caused primarily by a decrease in the C2sC3 π-bond order that occurs when the photon is absorbed, perhaps accompanied by an increase in the C3sC8 π-bond order. The key to the height of the barrier in such cases is the difference in electron density between the π orbitals on either side of the methyl group, the larger the difference in this density, the larger the barrier. V3 increases from 500 to 522 cm-1 on complexation of 3MI in its ground state, and from 300 to 331 cm-1 on complexation of 3MI in its excited S1 state. Thus, it appears that the dispersion interaction with the attached Ar enhances the differences in the π-electron density on either side of the methyl group. In agreement with this, we find a significantly larger percentage change in V3 in the S1 state (∼10%), compared to the S0 state (∼4%), reflecting the ∼0.05 Å decrease in the COM distance in the S1 state. That an electrostatic interaction between the rare gas atom and the “surface” to which it is attached is principally responsible for the observed variation in barrier heights also is consistent with the previously noted trend in V3 values in the MTN-rare gas complexes.30 1MI has smaller barriers in both states, V3(S0) ≈ 260 and V3(S1) ≈ 100 cm-1. Clearly, these reflect the expected smaller differences in the π-electron densities in the bonds adjacent to the indole nitrogen. Additionally, the preferred conformation of the methyl group (according to theory) is eclipsed with respect to N1sC2, rather than N1sC9. Apparently, this is a consequence of a repulsive (peri) interaction with the C7 hydrogen on the adjacent ring. The S1 barrier is substantially smaller in magnitude than the S0 barrier, possibly as a consequence of an increased separation of the two interacting groups. But, despite the magnitudes of these barriers, in both 1MI and 5MI, complexation with argon again increases their magnitude, though to a lesser extent. A less polarizable surface is not as perturbed by the attachment of an interacting atom to it. Parenthetically, it is interesting to note that the red shift observed on complexation of 1MI with Ar (16.6 cm-1) is substantially less than that for 3MI (31.8 cm-1) and 5MI (28.3 cm-1). 1MI also differs in other respects; its van der Waals bond with Ar is substantially shorter than the bonds in 3/5MIA in the S0 state, but only slightly shorter than 3/5MIA in the S1 state. The Watson centifugal distortion terms used to fit the spectrum of 1MIA are also significantly smaller than those for 3MIA. All of these data suggest that the Ar atom is more tightly bound in 1MIA than in the other indole complexes.
Korter and Pratt The photoinduced change in the position of the argon atom on going from the S0 to the S1 state is of the order of 0.05 Å, a typical (large) vibrational amplitude. This change in the position of the weakly bound atom produces substantial changes in the electronic wave function of the “surface” to which the atom is attached, which in turn lead to changes in its structure. Although it is difficult to quantify the magnitudes of these changes, it seems at least feasible that they could be modulated by such a motion. Thereby established is a direct link between the observed results and a proposed mechanism for vibrational relaxation on real surfaces.5 Finally, we find no evidence in any of our data for any solvent-induced mixing of the excited states of indole. The observed transition moment orientations in the spectra of all three complexes are all consistent with an 1Lb S1 state,11 and can be explained by a purely mass-induced rotations of the inertial axes. Summarizing, we have shown that the barrier to internal rotation of a methyl group attached to an unsaturated linkage in an organic molecule is changed by complex formation with a weakly bound argon atom. We have argued that this is a consequence of a dispersion interaction between the argon and the π-electrons of the molecule, especially in the vicinity of the methyl group. And we have suggested that future studies of such interactions will greatly improve our understanding of the nature of the couplings between adsorbate vibrations and the surface to which it is attached, especially those mediated by electrons. Acknowledgment. This work has been supported by the NSF (Grant No. CHE-9987048). References and Notes (1) Majewski, W. A.; Meerts, W. L. J. Mol. Spectrosc. 1984, 104, 271. (2) Pratt, D. W. Annu. ReV. Phys. Chem. 1998, 49, 481. (3) Tan, X.-Q.; Majewski, W. A.; Plusquellic, D. F.; Pratt, D. W.; Meerts, W. L. J. Chem. Phys. 1989, 90, 2521. (4) Yates, J. T., Jr.; Alvey, M. D.; Dresser, M. J.; Henderson, M. A.; Kiskinova, M.; Ramsier, R. D.; Szabo´, A. Science 1992, 255, 1397. (5) Persson, B. N. J.; Persson, M. Solid State Comm. 1980, 36, 175. (6) Majewski, W. A.; Pfanstiel, J. F.; Plusquellic, D. F.; Pratt, D. W. in Laser Techniques in Chemistry; Myers, A. B., Rizzo, T. R., Eds.; John Wiley & Sons: New York, 1995; p. 101. (7) Gerstenkorn, S.; Luc, P. Atlas du Spectroscopie d’Absorption de la Molecule d’Iode, CNRS, Paris, 1978 and 1982. (8) Spangler, L. H.; Pratt, D. W. in Jet Spectroscopy and Molecular Dynamics; Hollas, J. M., Phillips, D., Eds.; Chapman and Hall: London, 1995; p. 366. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A9; Gaussian, Inc.: Pittsburgh, PA, 1998. (10) Berden, G.; Meerts, W. L.; Jalviste, E. J. Chem. Phys. 1995, 103, 9596. (11) Korter, T. M.; Kuepper, J.; Pratt, D. W. J. Chem. Phys. 1999, 111, 3946. (12) Plusquellic, D. F. et al., to be published. (13) Watson, J. K. G. in Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1977; Vol. 6, p 1. (14) Gordy, W.; Cook, R. L. MicrowaVe Molecular Spectra, 3rd ed.; Wiley-Interscience: New York, 1984.
Internal Rotation of Attached Methyl Groups (15) Tan, X.-Q.; Majewski, W. A.; Plusquellic, D. F.; Pratt, D. W. J. Chem. Phys. 1991, 94, 7721. (16) Held, A.; Champagne, B. B.; Pratt, D. W. J. Chem. Phys. 1991, 95, 8732. (17) Leach, G. W.; Demmer, D. R.; Bickel, G. A.; Wallace, S. C. J. Chem. Phys. 1993, 99, 67. (18) Champagne, B. B.; Pfanstiel, J. F.; Pratt, D. W.; Ulsh, R. C. J. Chem. Phys. 1995, 102, 6432. (19) Remmers, K.; Jalviste, E.; Mistrik, I.; Berden, G.; Meerts, W. L. J. Chem. Phys. 1998, 108, 8436. (20) In fact, the fully resolved spectrum of 3MIA exhibits axis tilting, like that of indole-Ar (ref 11). A careful intensity fit of the data for 3MIA yields a hybrid band composed of 46 ( 3% a, 34 ( 3% b, and 20 ( 2% c character, with an axis tilt of θT ) ( 8.0 ( 1.0°. (21) Plusquellic, D. F.; Pratt, D. W. J. Chem. Phys. 1992, 97, 8970. (22) Bickel, G. A.; Leach, G. W.; Demmer, D. R.; Hager, J. W.; Wallace, S. C. J. Chem. Phys. 1988, 88, 1. (23) Sammeth, D. M.; Siewert, S. S.; Callis, P. R.; Spangler, L. H. J. Phys. Chem. 1992, 96, 5771. (24) Tan, X.-Q.; Sun, L.; Kuczkowski, R. L. J. Mol. Spectrosc. 1995, 171, 248. (25) Huber, S.; Makarewicz, J.; Bauder, A. Mol. Phys. 1998, 95, 1021. (26) Rottstegge, J.; Hartwig, H.; Dreizler, H. J. Mol. Spectrosc. 1999, 195, 1.
J. Phys. Chem. B, Vol. 105, No. 18, 2001 4017 (27) Velino, B.; Melandri, S.; Favero, P. G.; Dell EÄ rba, A.; Caminati, W. Chem. Phys. Lett. 2000, 316, 75. (28) Fraser, G. T.; Lovas, F. J.; Suenram, R. D. J. Mol. Spectrosc. 1994, 167, 231. (29) Alonso, J. L.; Lo´pez, J. C.; Blanco, S.; Lesarri, A.; Lorenzo, F. J. J. Chem. Phys. 2000, 113, 2760. (30) Takayanagi, M.; Hanazaki, I. J. Phys. Chem. 1996, 100, 10037. (31) Brumbaugh, D. V.; Kenney, J. E.; Levy, D. H. J. Chem. Phys. 1983 78, 3415. (32) Cheng, P.-Y.; Lapierre, L.; Ju, S.-S.; DeRose, P.; Dai, H.-L. Z. Phys. D. 1994, 31, 105. (33) See, for example: Sands, W. D., Jones, L. F., Moore, R. J. Phys. Chem. 1989, 93, 6601. W. E. Sinclair and D. W. Pratt, J. Chem. Phys. 1996, 105, 7942. (34) Ab initio calculations are in qualitative agreement with the ground state V3 values. A MP2/6-31G** relaxed potential energy scan of the methyl torsion yielded barrier heights of 428 cm-1 for 3MI and 322 cm-1 for 1MI. This level of theory is insufficient to reproduce the observed changes induced by complexation. (35) Hehre, W. J.; Pople, J. A.; Devaquet, A. J. P. J. Chem. Soc. 1976, 98, 664. (36) Dorigo, A. E.; Pratt, D. W.; Houk, K. N. J. Chem. Soc. 1987, 109, 6591.
