ARTICLE pubs.acs.org/JPCA
Absolute Intensities of NH-Stretching Transitions in Dimethylamine and Pyrrole Benjamin J. Miller,† Lin Du,‡ Thomas J. Steel,† Allanah J. Paul,† A. Helena S€odergren,† Joseph R. Lane,§ Bryan R. Henry,|| and Henrik G. Kjaergaard*,‡ †
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark § Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand Department of Chemistry, University of Guelph, N1G 2W1, Guelph, Ontario, Canada
)
‡
bS Supporting Information ABSTRACT: Vibrational spectra of vapor-phase dimethylamine (DMA) and pyrrole have been recorded in the 1000 to 13000 cm1 region using long path conventional spectroscopy techniques. We have focused on the absolute intensities of the NH-stretching fundamental and overtone transitions; ΔνNH = 14 regions for DMA and the ΔνNH = 13 regions for pyrrole. In the ΔνNH = 13 regions for DMA, evidence of tunneling splitting associated with the NH-wagging mode is observed. For DMA, the fundamental NH-stretching transition intensity is weaker than the first NH-stretching overtone. Also, the fundamental NH-stretching transition in DMA is much weaker than the fundamental transition in pyrrole. We have used an anharmonic oscillator local mode model with ab initio calculated local mode parameters and dipole moment functions at the CCSD(T)/ aug-cc-pVTZ level to calculate the NH-stretching intensities and explain this intensity anomaly in DMA.
’ INTRODUCTION The near-infrared (NIR) region of the electromagnetic spectrum is dominated by XH-stretching (where X is O, C, N, S, etc.) vibrational overtone transitions.13 These transitions are sensitive to both the physical and chemical environment of the chemical bond being probed. Measured absolute intensities in turn are sensitive to both potential energy curves and dipole moment functions. Fundamental and overtone absolute intensities of XH-stretching transitions have been previously reported for a number of molecules (see, for example, refs 27 and references therein). Vibrational coupling complicates spectra and makes experimental intensities of the XH-stretching transitions more difficult to determine.811 There have been a number of studies on CH- and OH-oscillators, less on NH- and very few on SH-oscillators, which is related to the increasing difficulty in recording the spectra. For CH- and OHstretching vibrations, the intensities are strongest for the fundamental transition and typically decrease by about one order of magnitude with each subsequent level of excitation.4,6,12 However, for a few molecules, the XH-stretching transitions show a different behavior with the fundamental transition intensity similar to or weaker than the intensity for the first overtone.1315 To facilitate comparison between NH-, OH-, and SH-stretching oscillators and to limit vibrational coupling, we investigated dimethylamine (DMA) and pyrrole (Figure 1) as examples of stable molecules r 2011 American Chemical Society
that contain an NH group. DMA and pyrrole are particularly interesting as a pair of NH-containing molecules in that they display different intensity trends with increasing in vibrational excitation. There have been numerous previous reports on the vibrational spectrum of pyrrole.1638 The majority of these are experimentally based studies. However, none of these studies give absolute intensities for the vibrational overtones. There have been fewer reports on the vibrational spectrum of DMA.3941 The most important related to the current work is that of Fang et al. who used intracavity photoacoustic spectroscopy (PAS) to measure overtone absorptions of DMA.39 The photoacoustic signal is proportional to absorbance with an unknown constant.42 Therefore no absolute absorbance data are usually obtained from PAS experiments. We study the differences in the spectra of these two molecules and explain them on the basis of an anharmonic oscillator local mode model with ab initio calculated dipole moment functions obtained with coupled cluster theory.
’ EXPERIMENTAL SECTION Gaseous anhydrous dimethylamine (Aldrich, g 99%) was loaded into the respective sample cells on a vacuum line without Received: September 21, 2011 Revised: November 9, 2011 Published: November 10, 2011 290
dx.doi.org/10.1021/jp209118p | J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
any further purification. Pyrrole (Aldrich, g 98%) was dried and degassed with several freezepumpthaw cycles on the vacuum line. All vapor pressures were monitored using a high vacuum diaphragm manometer (Varian DV100) coupled to a pressure gauge (Varian model 654325039). Details of each of the experiments are summarized in Tables S1 and S2 of the Supporting Information (SI). Room temperature spectra of the ΔνNH = 13 transitions of both species were measured in the region of 1000 to 11000 cm1 with a 10 cm path length cell or a multipass cell (Infrared Analysis Inc.) with a path length of either 2.4 or 4.8 m. The spectra were recorded on a Perkin Elmer Spectrum 100 FTIR spectrometer with a quartz/halogen source, a KBr beamsplitter, and an FRDTGS detector or a Bruker Vertex 70 with an MIR or NIR source, a CaF2 beamsplitter, and a liquid N2 cooled MCT or InGaAs detector. The ΔνNH = 4 spectra of DMA were recorded with the 4.8 m path length on a Varian Cary 500 spectrophotometer. The Cary spectra were recorded with the spectrometer operating in double beam mode. A neutral density filter, with an optical density of 0.12 was placed in the reference beam. The ΔνNH = 4 spectra of DMA were recorded at a spectral bandwidth of 0.75 nm (12 cm1 at 12500 cm1) and a data interval of 0.25 nm, with a 3 s averaging time. Eight scans were then coadded to reduce noise. The Grams or OPUS software packages were used to obtain the integrated absorbance using a straight baseline. We deconvoluted the overlapping spectra of DMA and pyrrole into Lorentzian bands or identify the peak center from the location of the Q-branch.39,43 Absolute Intensities. To improve experimental accuracy, we recorded NH-stretching spectra of the same species with differing sample vapor pressures and path lengths. The range of intensities thus obtained provides an indication of experimental error. Oscillator strengths were obtained using the following equation:44 Z T Aðν~Þdν~ ð1Þ f ¼ 2:6935 109 ðK 1 Torr m cmÞ pl where p is the sample pressure in the cell, l is the path length of the cell, T is the temperature, and A is the absorbance (log10). We have found this method to give reproducible OH-stretching overtone intensities for ethanol that are in good agreement with previously published values.6,45 The absolute intensities are found from the slope of a plot of the integrated intensity times temperature versus path length times vapor pressure. The slope is multiplied by 2.6935 109 K1 Torr m cm to give the oscillator strength. The quoted uncertainties are from a two-tailed t test at 95% probability using the required degrees of freedom.
’ THEORY AND CALCULATIONS We have optimized the geometries of DMA (Cs) and pyrrole (C2v) with the coupled cluster with single, double and perturbative triples method [CCSD(T)] and the aug-cc-pVTZ correlation consistent basis sets and with the B3LYP/aug-cc-pVTZ method. The optimization threshold criteria were set to the following: gradient = 1 106 a.u.; stepsize = 1 106 a.u.; energy = 1 108 a.u. We have calculated the NH-stretching fundamental and overtone transitions of DMA and pyrrole with an anharmonic oscillator (AO) local mode model.3,46 We assume that the isolated
NH-stretching vibrational mode can be described by a Morse oscillator, where the vibrational energy levels are given by 1 ~ 1 2~ ωx ð2Þ ω v þ EðvÞ=ðhcÞ ¼ v þ 2 2 The Morse oscillator frequency ω~ and anharmonicity ω~x were calculated from the second, third, and fourth order derivatives of the potential energy curve as described previously.46,47 These derivatives were found by fitting a 12th-order polynomial to a 13point ab initio potential energy curve, which was obtained by displacing the NH-bond from 0.30 to +0.30 Å in 0.05 Å steps about equilibrium. This range and stepsize ensured converged energy derivatives.48 The dimensionless oscillator strength f of a transition from the vibrational ground state |0æ to a vibrationally excited state |νæ is given by8,44 fv, 0 ¼ 4:702 107 cm D2 ~vv, 0 jμ ~v, 0 j2 ð3Þ where ν~ν,0 is the transition wavenumber (cm1) and μBν,0 = Æν|μB|0æ is the transition dipole moment matrix element in debye (D). We expand the transition dipole moment matrix element as Ævjμj0æ ¼
∂μ 1 ∂2 μ 1 ∂3 μ Ævjqj0æ þ Ævjq2 j0æ þ Ævjq3 j0æ þ ::: 2 ∂q 2 ∂q 6 ∂q3
ð4Þ where q is the internal vibrational coordinate. The integrals Æν|qn|0æ required for the transition dipole moment were evaluated analytically.49 The dipole moment derivatives in eq 4 were found by fitting a sixth-order polynomial to a 15-point dipole moment curve, which was obtained by displacing the NH-bond from 0.30 to +0.40 Å in 0.05 Å steps about equilibrium.48 The CCSD(T) dipole moment was calculated from a finite field approach with a field strength of (0.0005 a.u. The single point CCSD(T) threshold criteria were set to the following: energy = 1 109 a.u.; orbital = 1 108 a.u.; coeff = 1 108 a.u. All calculations assumed a frozen core (C, 1s; N, 1s) and were performed with MOLPRO.50 The normal mode harmonic frequencies were calculated with the B3LYP/aug-cc-pVTZ method and are given in Tables S3 and S4 of the SI. We have also calculated harmonic vibrational frequencies of DMA and pyrrole with the CCSD(T)/aug-ccpVTZ method using analytical derivatives in CFOUR and present these in Tables S5 and S6 of the SI.51 The single point and optimization convergence threshold criteria for the CFOUR calculations were set to the following: SCF_CONV = 1 1010 a.u., CC_CONV = 1 1011 a.u., LINEQ_CONV = 1 1011 a.u., and GEO_CONV = 1 1010 a.u.
’ RESULTS AND DISCUSSION We present selected CCSD(T)/aug-cc-pVTZ structural parameters for DMA and pyrrole in Tables S7 and S8 of the SI, respectively. We find our calculated parameters to be in good agreement with the structural parameters derived from microwave and electron diffraction experiments.5254 The NH-bond length in DMA is 0.01 Å longer than that in pyrrole, and this is reflected in the observed stretching frequency being much smaller in DMA. Pyrrole is planar, whereas DMA has a pyramidal structure. This leads to the NH-bond in DMA being able to flip from one side to the other in a wagging mode.19,55 This mode has a double well potential that leads to the observed tunneling 291
dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
Figure 1. Structures of DMA (right) and pyrrole (left).
splitting of the NH-stretching transitions, similar to what has been observed in species such as H2O2 and aniline.5658 NH-Stretching Transitions. The ΔνNH = 14 transitions of DMA are shown in Figure 2 and the transition wavenumbers and intensities are given in Table 1. Our observed ΔνNH = 1 and 2 transitions are in good agreement with the previous experimental investigations.40,59 The NH-stretching bands for DMA show significant overlap with spectral features that previously were attributed to hot bands of a low frequency mode.39 We assign these double bands in each of the ΔνNH = 13 transitions as arising from tunneling splitting of the NH-stretching energy levels due to the double well potential of the NH-wagging mode. The separation of the two observed bands depends on the difference in tunneling splitting of the NH-stretching ground state and of the NH-stretching excited state. This is similar to what has been observed in aniline, where the tunneling splitting of the NH-stretching energy levels was found to decrease as higher NH-stretching transitions were excited.57,58 This led to two bands observed in each vibrational NH-stretching transition. In DMA, we observe Q-branches at the center of these two bands for both the fundamental and first overtone NH-stretching transition. For these we find that the bands are split by 15 and 12 cm1 for the fundamental and first overtone, respectively, with a ∼17 cm1 splitting for the second overtone. The splitting of these bands does not indicate a clear pattern in the change of tunneling splitting of the associated levels as higher NH-stretching transitions are excited. In the ΔνNH = 4 region, we observe two well separated strong bands at 12432 and 12594 cm1 with oscillator strengths around 1 109. The higher energy band has a shoulder about 40 cm1 lower in energy, which could be due to tunneling as seen in the lower energy transitions. The strong lower energy band at 12432 cm1 likely arises from resonance coupling between the pure ΔνNH = 4 stretch and a combination band (see Supporting Information). A likely candidate for the resonance is a state that involves three quanta of NH-stretching and one quantum of CHstretching. The energy match is good if we use our calculated CH-stretching harmonic frequencies at the B3LYP/aug-ccpVTZ level (B3LYP harmonic frequencies are close to the observed frequencies). Moreover, the CH-stretching fundamental transition is intrinsically strong, and thus, the combination is possibly also strong. Such coupling between vibrations that do not share a common atom has been observed previously in formic acid and methanol.11,60 Slightly to the red of the ΔνNH = 2 region, a band with about one-third of the intensity of the fundamental NH-stretching transition is observed at 6225 cm1 (shown in Figure S1 of the SI). We assign this band to the NH-stretching plus CHstretching combination. We observe the perturbed bands at 12432 and 12594 cm1 with approximately equal oscillator strengths. In the Supporting
Figure 2. Room temperature vapor phase overtone spectra of DMA in the ΔνNH = 14 regions. The ΔνNH = 1 spectrum was recorded at a pressure of 20 Torr and a path length of 4.8 m on the Bruker Vertex 70 FTIR with 1 cm1 resolution. The ΔνNH = 2 spectrum was recorded at a pressure of 498 Torr and a path length of 9.6 cm on the PE100 FTIR with 1 cm1 resolution. The ΔνNH = 3 spectrum was recorded at a pressure of 142 Torr and a path length of 4.8 m on the Bruker Vertex 70 FTIR with 1 cm1 resolution. The ΔνNH = 4 spectrum was recorded at a pressure of 496 Torr and a path length of 4.8 m on the Cary 500 with a spectral bandwidth of 0.75 nm.
Information we calculate the transition wavenumber of the unperturbed states to be ν~1 = 12508 cm1 and ν~II = 12518 cm1, respectively. However, taking into account the tunneling splitting, this changes to ν~1 ∼ 12494 cm1 and ν~II ∼ 12530 cm1. The harmonic B3LYP/aug-cc-pVTZ frequency of the strongest CH-stretching transition is around 2842 cm1. In the ΔνNH = 4 region, the equivalent combination band with three quanta in the NH-stretching transition would be expected around 12498 cm1. In measuring the intensity of the ΔνNH = 4 stretching transition in 292
dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
Table 1. Calculated and Observed NH-Stretching Wavenumbers (cm1) and Oscillator Strengths (f) of DMA ΔvNH 1
νobs 3359b
fobs
νcalcda
fcalcda
(1.01 ( 0.13) 107
3368
1.10 107
(5.49 ( 0.90) 107
6584
3.78 107
(1.77 ( 0.46) 108
9648
1.98 108
(1.43 ( 0.46) 109
12559
1.12 109
b
2
3374 6579b,c b
6591 3
9639b d
9656 4
12432d,e 12594
d
a
Calculated with the local mode anharmonic oscillator model. Dipole moment function and local mode parameters at the CCSD(T)/augcc-pVTZ level of theory. b The peak position of the transition is taken as the position of the Q-branch. c Observed at 6580 cm1 in Marinov et al.40 d Energies taken from Lorentzian deconvolutions. e Likely contribution from local mode combination bands and possible resonance.
DMA, we have integrated the entire resonant band structure as we assume that all the intensity was from the “bright” pure NHstretching mode in the zero-order system. The NH-stretching frequencies of DMA calculated with the AO local mode model agree well with observed ΔνNH = 13 transitions (higher energy transition in each doublet) with the largest difference being 8 cm1. We assign the most intense and highest energy transition in each tunnel split doublet to the transition arising from the wagging ground state. The ΔνNH = 13 transitions for pyrrole are shown in Figure 3 and the transition wavenumbers and intensities are given in Table 2. Well-resolved P, Q, and R branches are observed in the spectra of the ΔνNH = 13 transitions. Our spectra look similar to the previously reported NH-stretching overtone spectra of pyrrole by Snavely et al.18 The NH-stretching wavenumbers of pyrrole calculated with the AO local mode model agree quite well with the observed ΔνNH = 13 transitions with the largest difference being 20 cm1. A weak band to the low energy side of the pure NH-stretching band is clearly observed in the ΔνNH = 2 and 3 spectra. Snavely et al. did not venture an assignment for this lower energy band. However, they did record vibrational overtone spectra of pyrrole-d4 (four deuterium atoms on the carbon atoms) and observed no shift in the lower energy bands. The absence of a shift rules out any contribution byCH-stretching modes. Zumwalt and Badger made one of the earliest studies on this low energy band.17 They observed the ΔνNH = 3 spectrum of pyrrole at 150 and 250 C and found the lower energy band to increase markedly in intensity at the higher temperature. They accordingly assigned this lower energy band as a hot band of an NH-bend that they estimated had a fundamental transition wavenumber of ∼650 cm1.17 Hassoon and Snavely observed the change in intensity of the hot band with increasing temperature in the ΔνNH = 4 region of pyrrole and estimated the absorption originated around 340 cm1 above the vibrational ground state.19 They assign the low energy band as a hot band originating from the NHwagging, which has a fundamental transition wavenumber of 474 cm1.19,55 NH-Stretching Absolute Intensities. We present the absolute intensities for the NH-stretching transitions of DMA and pyrrole in Tables 1 and 2, respectively. We have recorded the regions at multiple combinations of pressure and path lengths
Figure 3. Room temperature vapor phase overtone spectrum of pyrrole in the ΔνNH = 13 regions. The ΔνNH = 1 spectrum was recorded at a pressure of 1 Torr and a path length of 2.4 m on the PE100 FTIR with 1 cm1 resolution. The ΔνNH = 2 and 3 spectra were recorded at a pressure of 5.3 Torr and a path length of 4.8 m on the Bruker Vertex 70 FTIR with 1 cm1 resolution.
to estimate the uncertainty in the experimentally determined intensities. In measuring the intensity of the ΔνNH = 14 stretching transition in DMA, we have integrated the entire band structure. We believe this to be accurate in the ΔνNH = 13 regions, however, the ΔνNH = 4 region is likely to have contribution from combination bands. In pyrrole we have also integrated the entire band for all regions. The measured intensities are overall in good agreement with the AO calculated 293
dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
Table 2. Calculated and Observed NH-Stretching Wavenumbers (cm1) and Oscillator Strengths (f) of Pyrrole ΔvNH 1b
fobs
3531 6873 6924
b
fcalcda
3.3 106
3541
8.53 106
(4.0 ( 0.5) 107
6942
3.64 107
10105
1.1 108
10204
8.38 109
13328
2.35 1010
c
10184 4
νcalcda
c
2 3
νobs
13305
d
a
Calculated with the local mode anharmonic oscillator model. Dipole moment function and local mode parameters at the CCSD(T)/augcc-pVTZ level of theory. b Too few data for uncertainty estimate. c Weak combination or hot band. d Taken from Snavely et al.18 Figure 4. ΔνNH = 1 (left) and ΔvNH = 2 transitions of DMA. Notice the unusual relative intensity.
Table 3. Calculated and Observed Local Mode Parameters (cm1) for DMA and Pyrrolea DMA calculated
a
a
pyrrole b
observed
Fang
calculated
a
observed
ω ~
3520.6
3529.1 ( 1.0
3530
3679.6
3667.6 ( 0.3
ω ~x
76.2
77.7 ( 0.5
81
69.5
68.3 ( 0.1
Calculated at CCSD(T)/aug-cc-pVTZ. b Taken from Fang et al.39
intensities. For all but the ΔνNH = 1 transition in pyrrole the agreement is within a factor of 2. The CCSD(T)/aug-cc-pVTZ calculated and experimentally derived local mode parameters are given in Table 3. The calculated values for ω~ and ω ~x are smaller compared with experiment for DMA and larger for pyrrole. However, the error is only about (10 cm1 for ω~ and less than (2 cm1 for ω~x. These differences are consistent with what have been observed previously with CCSD(T)/aug-cc-pVTZ calculated local mode parameters for both OH- and SH-oscillators in other molecules.45,46,61,62 Fang et al. measured the absorption spectra of DMA up to the fifth CH- and NH-stretching overtone and noticed that in the NIR and visible spectral region, the NH-stretching overtone progression shows absorption strength comparable to that observed for the corresponding CH-stretching overtones. However, in the fundamental region the strength of the NH-stretching absorption was found to be very low.39 The fundamental NHstretching transition is at the tail of the strong CH-stretching band. The spectrum showing the relative intensities of CH- and NH-stretching fundamental bands is given in Figure S2 of the SI. The fundamental NH-stretching vibration of DMA is not only weak in comparison with the fundamental CH-stretching vibrations. The intensity of the first NH-stretching overtone is also stronger than the fundamental NH-stretching by a factor of ∼5, as shown in Figure 4. This unusual behavior is understood through an examination of eq 4.63 The integral Æ1|q|0æ is about 20 times larger than Æ1|q2|0æ for an NH-stretching oscillator and thus fundamental NH-stretching transitions normally derive their intensity primarily from the first term in eq 4.64 In other words, the intensity of a fundamental transition is proportional to the derivative of the dipole moment function (DMF) around the equilibrium bond length (q = 0). For DMA this dipole moment derivative happens to be very close to zero and hence the intensity is very small. In Figure 5 we show the DMF for DMA and
Figure 5. Comparison of the dipole moment functions for the NH-stretch of DMA (top) and pyrrole (bottom).
pyrrole, which clearly illustrates this difference in DMFs. The expansion coefficients of the DMF are given in Tables S9 and S10 of the SI. The DMF for DMA is very flat around the equilibrium bond length (note the vertical axis scale) when compared to pyrrole, which has a much greater slope. Hence, the fundamental intensity for the NH-stretching mode of DMA is much lower than the NH-stretching mode of pyrrole. The intensity of the first overtone depends almost equally on the first two terms in the expansion eq 4 and thus the curvature of the DMFs become 294
dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
transitions significantly weaker and SH-stretching transitions very weak. Part of the reason for this is the decreasing anharmonicity from OH to NH to CH to SH, but also the location of the equilibrium bond length on the dipole moment curve. For OHbonds the equilibrium bond length is located before the extremum on the DMF curve while for the longer CH-bond it is usually after the extremum. For NH-bonds the equilibrium bond length is very close the extremum on the DMF and hence for a few molecules the dipole derivative becomes close to zero.13,62
Figure 6. Comparison of measured NH-, OH-, CH-, and SH-stretching absolute intensities. Intensities for OH-, CH-, and SH-stretching transitions are taken from ref 45.
important. From inspection of the second derivatives it is not surprising that the first overtone intensity of the two molecules is comparable. This anomalous intensity pattern has also been found in a few other examples. In cyclopropylamine the fundamental NH-stretching intensity was found to be similar to the intensity of the first NH-stretching overtone15 and in chloroform where the CH-stretching first overtone is about five times more intense than the fundamental CH-stretching transition.13,14 In the ΔνNH = 1 region, contribution to the total intensity is possible from DMA dimer formation. Fang and Swofford did not observe any DMA dimer formation for pressures up to 100 Torr.39 However, recently the dimer has been observed with an estimated equilibrium constant Kp of around 1.4 103 atm1 at room temperature.65 The combination of the very weak NHstretching fundamental and the large intensity enhancement by dimer formation, leads to a significant intensity contribution from dimer formation at high pressures. However, as dimer concentration scales with the pressure squared this is minimized by recording spectra at lower pressures. For the overtones, there is limited intensity enhancement by dimer formation and higher pressures can be used.65,66 For pyrrole, the calculated intensity of the ΔνNH = 1 transition is approximately a factor of 3 larger than the measured intensity. For the ΔνNH = 2 and 3 transitions, the calculated intensities are 10 and 24% smaller, respectively. We expect the pyrrole results to have relatively large uncertainty due to the lower vapor pressure and more importantly the strong tendency pyrrole has to adsorb to sample cell walls, which prevents accurate determination of the sample pressure. In several experiments we found condensation of liquid pyrrole in our gas cells. For the ΔνNH = 1 transitions of pyrrole we were unable to get reliable vapor pressure measurements with a 10 cm cell as the silicon glue used to attach the windows to the cell body appeared to be adsorbing pyrrole during the experiment. Use of long path cells for the fundamental transition was similarly fraught with uncertainty as we had to use very small vapor pressures as to not saturate the spectrum. In Figure 6, we compare measured NH-stretching, OH-, CH-, and SH-stretching intensities.6,45 Apart from DMA, all the molecules displayed show the usual drop in intensity with increasing vibrational number. The NH-stretching intensities are comparable to OH-stretching intensities, with CH-stretching
’ CONCLUSION The ΔνNH = 14 transitions of DMA and the ΔνNH = 13 transitions of pyrrole have been recorded using long path conventional spectroscopic techniques. We have obtained absolute NH-stretching transition intensities from these spectra and compared with calculated absolute intensities from an anharmonic oscillator local mode model with CCSD(T)/aug-cc-pVTZ local mode parameters and dipole moment functions. The agreement is excellent for DMA and good for pyrrole where we believe that the tendency of pyrrole to condense in the sample cell makes recording accurate intensities difficult. We find that the fundamental NH-stretching transition of DMA has a very low intensity compared to the fundamental transition of pyrrole. Moreover, the fundamental DMA intensity is significantly lower than that of its first overtone. We explain this result on the basis of the shape of the NH-stretching dipole moment function and the fact that for DMA the equilibrium NH-bond length is close to the extremum on the dipole moment function. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional spectra recorded for DMA in the 26007000 cm1 region. Tables with details of all experiments included for DMA and pyrrole. Harmonic oscillator calculated harmonic frequencies and intensities for DMA and pyrrole. Optimized structural parameters for DMA and pyrrole. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: 45-35320322. Phone: 45-35320334.
’ ACKNOWLEDGMENT We acknowledge funding for this project from the Marsden Grant, administered by the Royal Society of New Zealand and Danish Council for Independent ResearchNatural Sciences. T. J.S. and A.J.P. gratefully acknowledge funding from the lasers and applications research theme (LART) at the University of Otago. B.J.M. acknowledges funding from the Division of Sciences, University of Otago. ’ REFERENCES (1) (2) (3) (4) (5) 295
Henry, B. R. Acc. Chem. Res. 1977, 10, 207. Henry, B. R. Acc. Chem. Res. 1987, 20, 429. Henry, B. R.; Kjaergaard, H. G. Can. J. Chem. 2002, 80, 1635. Quack, M. Annu. Rev. Phys. Chem. 1990, 41, 839. Halonen, L. Adv. Chem. Phys. 1998, 104, 41. dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
The Journal of Physical Chemistry A
ARTICLE
(48) Rong, Z.; Kjaergaard, H. G. J. Phys. Chem. A 2002, 106, 6242. (49) Rong, Z.; Cavagnat, D.; Lespade, L. Lecture Notes in Computer Science; Springer-Verlag: Berlin, 2003; Vol. 2658. (50) Werner, H.-J.; Knowles, P. J.; Manby, F. R.; Sch€utz, M.; Celani, P.; Knizia, G.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.; Adler, T. B.; Amos, R. D.; Bernhardsson, A.; Berning, A.; Cooper, D. L.; Deegan, M. J. O.; Dobbyn, A. J.; Eckert, F.; Goll, E.; Hampel, C.; Hesselmann, A.; Hetzer, G.; Hrenar, T.; Jansen, G.; K€ oppl, C.; Liu, Y.; Lloyd, A. W.; Mata, R. A.; May, A. J.; McNicholas, S. J.; Meyer, W.; Mura, M. E.; Nicklass, A.; Palmieri, P.; Pfl€uger, K.; Pitzer, R.; Reiher, M.; Shiozaki, T.; Stoll, H.; Stone, A. J.; Tarroni, R.; Thorsteinsson, T.; Wang, M.; Wolf, A. MOLPRO, version 2010.1, a package of ab initio programs; 2010. (51) Stanton, J. F.; Gauss, J.; Harding, M. E.; Szalay, P. G., with contributions from Auer, A. A.; Benedikt, U.; Berger, C.; Bernholdt, D. E.; Bomble, Y. J.; Cheng, L.; Christiansen, O.; Heckert, M.; Heun, O.; Huber, C.; Jagau, T.-C.; Jonsson, D.; Juselius, J.; Klein, K.; Lauderdale, W. J.; Matthews, D. A.; Metzroth, T.; O’Neill, D. P.; Price, D. R.; Prochnow, E.; Ruud, K.; Schiffmann, F.; Schwalbach, W.; Stopkowicz, S.; Tajti, A.; Vazquez, J.; Wang, F.; Watts, J. D. and the integral packages MOLECULE (Alml€of, J. and Taylor, P. R.), PROPS (Taylor, P. R.), ABACUS (Helgaker, T., Jensen, H. J. Aa., Jørgensen, P., and Olsen, J.), and ECP routines by Mitin, A. V. and van W€ullen, C. CFOUR, coupledcluster techniques for computational chemistry; 2010. For the current version, see http://www.cfour.de. (52) Wollrab, J. E.; Laurie, V. W. J. Chem. Phys. 1968, 48, 5058. (53) Beagley, B.; Hewitt, T. G. Trans. Faraday. Soc. 1968, 66, 2561. (54) Nygaard, L.; Nielsen, J. T.; Kirchheiner, J.; Maltesen, G.; Rastrup-Andersen, J.; Sorensen, G. O. J. Mol. Struct. 1969, 3, 491. (55) Navarro, R.; Orza, J. M. Anal. Quim. 1983, 79, 557. (56) Kjaergaard, H. G.; Goddard, J. D.; Henry, B. R. J. Chem. Phys. 1991, 95, 5556. (57) Howard, D. L.; Robinson, T. W.; Fraser, A. E.; Kjaergaard, H. G. Phys. Chem. Chem. Phys. 2004, 6, 719. (58) Fehrensen, B.; Hippler, M.; Quack, M. Chem. Phys. Lett. 1998, 298, 320. (59) Bernard-Houplain, M.-C.; Sandorfy, C. Chem. Phys. Lett. 1974, 27, 154. (60) Boyarkin, O. V.; Lubich, L.; Settle, R. D. F.; Perry, D. S.; Rizzo, T. R. J. Chem. Phys. 1997, 107, 8409. (61) Lane, J. R.; Kjaergaard, H. G.; Plath, K. L.; Vaida, V. J. Phys. Chem. A 2007, 111, 5434. (62) S€odergren, A. H. Calculated Absolute Intensities of Vibrational Local Modes; Otago University: New Zealand, 2010. (63) Kjaergaard, H. G.; Low, G. R.; Robinson, T. W.; Howard, D. L. J. Phys. Chem. A 2002, 106, 8955. (64) Kjaergaard, H. G.; Henry, B. R. J. Chem. Phys. 1992, 96, 4841. (65) Du, L.; Kjaergaard, H. G. J. Phys. Chem. A 2011, 115, 12097. (66) Howard, D. L.; Kjaergaard, H. G. J. Phys. Chem. A 2006, 110, 9597.
(6) Lange, K. R.; Wells, N. P.; Plegge, K. S.; Phillips, J. A. J. Phys. Chem. A 2001, 105, 3481. (7) Hazra, M. K.; Sinha, A. J. Phys. Chem. A 2011, 115, 5294. (8) Kjaergaard, H. G.; Yu, H. T.; Schattka, B. J.; Henry, B. R.; Tarr, A. W. J. Chem. Phys. 1990, 93, 6239. (9) Ha, T.-K.; Lewerenz, M.; Marquardt, R.; Quack, M. J. Chem. Phys. 1990, 93, 7097. (10) Kjaergaard, H. G.; Turnbull, D. M.; Henry, B. R. J. Phys. Chem. A 1997, 101, 2589. (11) Howard, D. L.; Kjaergaard, H. G. J. Chem. Phys. 2004, 121, 136. (12) Rong, Z.; Henry, B. R.; Robinson, T. W.; Kjaergaard, H. G. J. Phys. Chem. A 2005, 109, 1033. (13) Robinson, C. C.; Tare, S. A.; Thompson, H. W. Proc. R. Soc. A 1962, 269, 492. (14) Tarr, A. W.; Zerbetto, F. Chem. Phys. Lett. 1989, 154, 273. (15) Niefer, B. I.; Kjaergaard, H. G.; Henry, B. R. J. Chem. Phys. 1993, 99, 5682. (16) Wulf, O. R.; Liddel, U. J. Am. Chem. Soc. 1935, 57, 1464. (17) Zumwalt, L. R.; Badger, R. M. J. Chem. Phys. 1939, 7, 629. (18) Snavely, D. L.; Blackburn, F. R.; Ranasinghe, Y.; Walters, V. A.; Gonzalez del Riego, M. J. Phys. Chem. 1992, 96, 3599. (19) Hassoon, S.; Snavely, D. L. J. Chem. Phys. 1993, 99, 2511. (20) Mellouki, A.; Lievin, J.; Herman, M. Chem. Phys. 2001, 271, 239. (21) Douketis, C.; Reilly, J. P. 1992. (22) Barone, V. Chem. Phys. Lett. 2004, 383, 528. (23) Burcl, R.; Handy, N. C.; Carter, S. Spectrochim. Acta A 2003, 59, 1881. (24) Geidel, E.; Billes, F. J. Mol. Struct.: THEOCHEM 2000, 507, 75. (25) Hegelund, F.; Larsen, R. W.; Palmer, M. H. J. Mol. Spectrosc. 2008, 252, 93. (26) Held, A.; Herman, M. Chem. Phys. 1995, 190, 407. (27) Kofranek, M.; Kovar, T.; Karpfen, A.; Lischka, H. J. Chem. Phys. 1992, 96, 4464. (28) Lord, R. C., Jr.; Miller, F. A. J. Chem. Phys. 1942, 10, 328. (29) Lubich, L.; Oss, S. J. Chem. Phys. 1997, 106, 5379. (30) Mellouki, A.; Auwera, J. V.; Herman, M. J. Mol. Spectrosc. 1999, 193, 195. (31) Mellouki, A.; Georges, R.; Herman, M.; Snavely, D. L.; Leytner, S. Chem. Phys. 1997, 220, 311. (32) Roos, B. O.; Malmqvist, P.; Molina, V.; Serrano-Andres, L.; Merchan, M. J. Chem. Phys. 2002, 116, 7526. (33) Tai, J. C.; Yang, L.; Allinger, N. L. J. Am. Chem. Soc. 1993, 115, 11906. (34) Eloranta, J. K. Z. Naturforsch., A 1970, 25, 1296. (35) Orza, J. M.; Escribano, R. J. Chem. Soc., Faraday Trans. 2 1985, 81, 653. (36) Xie, Y.; Fan, K.; Boggs, J. E. Mol. Phys. 1986, 58, 401. (37) Scott, D. W. J. Mol. Spectrosc. 1971, 37, 77. (38) Lee, S. Y.; Boo, B. H. J. Phys. Chem. 1996, 100, 15073. (39) Fang, H. L.; Swofford, R. L.; Compton, D. A. C. Chem. Phys. Lett. 1984, 108, 539. (40) Marinov, D.; Rey, J. M.; Muller, M. G.; Sigrist, M. W. Appl. Opt. 2007, 46, 3981. (41) Graffeuil, M.; Labarre, J.-F.; Leibovici, C. J. Mol. Struct. 1974, 22, 97. (42) Schattka, B. J.; Turnbull, D. M.; Kjaergaard, H. G.; Henry, B. R. J. Phys. Chem. 1995, 99, 6327. (43) Grams/AI, 8.0th ed.; ThermoFisher Scientific Inc.: Waltham, MA, 2008. (44) Atkins, P. W.; Friedman, R. S. Molecular Quantum Mechanics, 3rd ed.; Oxford University Press: Oxford, 1997. (45) Miller, B. J.; Howard, D. L.; Lane, J. R.; Kjaergaard, H. G.; Dunn, M. E.; Vaida, V. J. Phys. Chem. A 2009, 113, 7576. (46) Howard, D. L.; Jorgensen, P.; Kjaergaard, H. G. J. Am. Chem. Soc. 2005, 127, 17096. (47) Herzberg, G. Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules; D. Van Nostrand Company, Inc.: Princeton, NJ, 1950. 296
dx.doi.org/10.1021/jp209118p |J. Phys. Chem. A 2012, 116, 290–296
