Infrared diode laser spectroscopy of N212C18O2

Journal of Molecular Spectroscopy 270 (2011) 66–69

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Infrared diode laser spectroscopy of N2A12C18O2 Toichi Konno, Shinya Yamaguchi, Yasushi Ozaki ⇑ Department of Chemistry, Faculty of Science, Josai University, Sakado, Saitama 350-0295, Japan

a r t i c l e

i n f o

Article history: Received 8 August 2011 In revised form 7 September 2011 Available online 22 September 2011 Keywords: N2AC18O2 Infrared spectroscopy Diode laser van der Waals complex

a b s t r a c t The high-resolution infrared spectrum of N2A12C18O2 has been observed in the m3 band (2314 cm1) region of 12C18O2 with diode laser absorption spectroscopy of pulsed molecular beam. The geometry of N2A12C18O2 is similar to N2A12C16O2, a T-shaped structure with the nitrogen molecular axis pointing towards the carbon atom. The geometrical parameters of the T-shaped ground-state structure are determined as RNcmC = 3.7285(5) Å and (90HNcmCO) = 6.85(3)°. The vibrational band origin of N2A12C18O2 corresponding to the m3 mode of 12C18O2 shows a shift of 0.52499(10) cm1 with respect to that of 12C18O2. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction The geometrical structures of numerous van der Waals (vdW) complexes have been studied by microwave (MW) and infrared (IR) spectroscopy. However, the isotopic differences in their structures have seldom been discussed. This isotope effect is mostly ascribed to the widely expanded wavefunctions demonstrating the zero-point oscillation of vdW modes. Therefore, the isotope effect obtained from high-resolution spectra is expected to provide significant information on the nature of vdW bonding. In addition, isotope effects on the band origin shifts in infrared spectra are expected to provide information on the intramolecular potential change by complex formation. In our recent IR studies of ArA12C18O2 [1], KrA12C18O2 [2], NeA12C18O2 and XeA12C18O2 [3], and (12C18O2)2 [4] using a diode laser and a pulsed molecular beam, we have analyzed the intramolecular and intermolecular potentials related to the complex formation and revealed isotope effect on the vibrationally averaged structure. One of the vdW complexes, N2AC16O2, was studied by a highresolution IR spectroscopy and a T-shaped equilibrium structure (C2v) for N2AC16O2 was determined [5]. Later, the ab initio calculation was made for N2AC16O2 by Venayagamoorthy and Ford [6] who were intended to provide the theoretical basis for the interpretation of the matrix isolation infrared spectra. Although reports on this fundamental complex were limited, microwave spectra of 14 N2ACO2, 15N14NACO2 and 14N15NACO2 have recently been measured and evidence for the inversion of the N2 is presented, indicating that the nonrigidity of the complex causes a pronounced large amplitude motion [7]. ⇑ Corresponding author. Fax: +81 49 271 7985. E-mail address: [email protected] (Y. Ozaki). 0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2011.09.004

The target system has been extended to the N2ACO2 isotopomers in the present study. The IR spectrum of N2A12C18O2 is observed in the region of 2311–2316 cm1, and the 16OA18O isotope effects on both the intermolecular Ncm(center of mass in N2 subunit) AC distance and the NcmACAO angle are determined. 2. Experimental The vdW complex, N2A12C18O2, was prepared by conventional pulse jet expansion of a 0.5% mixture of 18O-enriched CO2 (98.7% 18O) with 5% of N2 and 94.5% of Ne into a vacuum chamber at room temperature. A slit-type nozzle (general valve, series 9, 12.5  0.2 mm2) was used with a typical backing pressure of 100 kPa. The IR radiation from a diode laser was introduced directly to the chamber through a BaF2 window, since the liquid– nitrogen cooled Pb salt laser diodes (Laser Components, IR-2315GMP) worked on a single mode. The radiation was reflected just after the pulse nozzle 21 times by a pair of flat mirrors mounted to be nearly parallel. The nozzle was operated at 1 Hz with opening duration of 8 ms; a wavenumber range of typically 0.5 cm1 was repeatedly scanned at each pulse. The IR radiation, emitting from the chamber through an inlet window but at a different level, was detected by a liquid-nitrogen cooled InSb detector, and the output was directly amplified with a conventional DC amplifier. The amplified signal was then averaged with a digital oscilloscope over typically 1024 pulses. Both 12C16O2 [8] and 12C18O2 [9] monomer gases were used for calibration of the diode laser frequency. The pathway of IR radiation to the chamber was purged with dry nitrogen to avoid atmospheric CO2 absorption. Under this 0.5% 18O-enriched CO2 and backing pressure (100 kPa), NeA12C18O2 [3] was not observed.

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T. Konno et al. / Journal of Molecular Spectroscopy 270 (2011) 66–69

Table 1 Vibrational band origin m0 (cm1), rotational constants (MHz) and centrifugal distortion constants (kHz) for N2–CO2.

3. Results and discussion 3.1. Molecular constants for N2ACO2 R

A part of the observed spectrum is shown in Fig. 1. The Q0(J) and RR0(J) branches show the most prominent features for a near symmetric prolate top for T-shaped N2ACO2, appearing in the range of 2314.70–2315.35 cm1. The maximum absorption is 3.7% in the neighborhood of 2315.1 cm1 and the spectral line width is typically 0.0025 cm1. The rotational temperature of N2A12C18O2 is estimated to be 4 K. The observed spectrum is compared with that calculated by use of the estimated molecular constants under the assumption that N2A12C18O2 has the same structure as N2A12C16O2 [5]. The nuclear spin statistics permit only the even Ka levels in the ground vibrational state and odd Ka levels in the m3 excited state of 12C18O2; in fact, only the transitions with even K 00a and odd K 0a values were observed. This proves experimentally that N2A12C18O2 has C2v symmetry, i.e., T-shaped equilibrium structure. The wavenumbers of the assigned transitions are fitted to the band origin, m0, and the rotational terms with Watson’s asymmetric rotor Hamiltonian [10] with rotational constants A, B, and C for the lower and upper states and the centrifugal distortion constants, DJ, DJK, DK, dJ, and dK, which are assumed to be equal in the upper and lower vibrational states in the fitting procedures. The DJ, DJK and DK, constants of N2A12C18O2 are treated as adjustable parameters, whereas the dJ and dK constants are ignored, since their omissions do not lower the quality of the fit. Several unresolved spectral lines, especially in the Q-branch, are included in the fit, but their inclusion has no influence on the quality of the fit. A total of 81 transitions is assigned and fitted for N2A12C18O2; the derived molecular constants are listed together with those for N2A12C16O2 [5] in Table 1, where the standard deviation (1r) of the fit is 4.5  104 cm1. The DK constant for N2A12C18O2 is well determined, since the four different K 0a K 00a sub-bands are favorably observed, while that for 12 16 N2A C O2 is not determined [5]. As shown in Fig. 1, weak peaks due to (12C18O2)2 produced simultaneously are observed but excluded in the assignment of N2A12C18O2 transitions by use of the calculated spectrum of (12C18O2)2 [4]; the peak marked with the asterisk at 2315.0139 cm1 is not only assigned to these complexes, but also not assigned to the carrier gasA12C18O2 complex, NeA12C18O2 [3]. The nitrogen stretching vibration becomes weakly infrared active in this complex and in fact, Fredin et al. [11] observed the infrared

12C18 O 2

00 01

0000:R(0) 12

m0 A00 B00 C00 A0 B0 C0 DJ DJK DK a

N2–12C18O2

N2–12C16O2a

2314.57379(10) 10547.4(13) 1999.31(40) 1668.20(40) 10464.8(11) 1997.93(39) 1664.74(38) 9.2(18) 465(16) 665(80)

2349.627846(94) 11885.3(11) 2063.18(34) 1743.21(43) 11793.06(91) 2061.13(26) 1740.27(29) 4.2(24) 557(32) –

Ref. [5].

activity of the N–N stretch at 2328.1 cm1 in the spectrum of CO2 in solid nitrogen. However, the peak marked with the asterisk is highly unlikely to be due to the N–N stretch because of the small transition probability [7]. It seems that the peak can be assigned to a stack of Q branch peaks of (N2)2C18O2. Nesbitt and co-workers report an infrared spectrum of Ar2CO2, which shows a strong Q-branch near its band origin, and the obtained band-origin shift of the trimer, Dmt = 0.8983 cm1 [12], is almost twice the value of ArACO2 dimer, Dmd = 0.4701 cm1 [13]. Both N2ACO2 and ArACO2 have Tshaped structure [5,13], and so we can expect the Q-branch of (N2)2C18O2 to appear around twice the shift of N2AC18O2. If the peak with the asterisk is due to (N2)2C18O2, Dmt is calculated to be 0.965 cm1, which gives Dmt/Dmd = 1.838. This ratio is close to the Dmt/Dmd ratio for the Ar–CO2 and Ar2CO2 case, 1.911.

3.2. Observed isotope effects on the shift values The shift Dm0(18O) in the band origin of N2A12C18O2 with respect to the m3 mode of 12C18O2 monomer, m0 = 2314.04880(1) cm1 [9], is compared with the corresponding shift for N2A12C16O2 [5], Dm0(16O) in Table 2, where the ratio of Dm0(18O) to Dm0(16O) is also listed; their shifts and ratios for NeACO2 [3,13] and (CO2)2 [4,14], which show blue shifts analogously to N2ACO2, are also listed in Table 2. If harmonic force fields are assumed for CO2 and CO2 complexes, and if the antisymmetric mode is well separated from the intermolecular modes of CO2 complexes, this ratio should be identical for any CO2 complexes and equal to that for the monomer band origins, 0.9851 [1–4]. However, the ratio for N2ACO2 as well as NeACO2 [3] exceeds unity; these values look anomalous because the vibrational motion of a heavier isotope should have a lower frequency.

C18 O2

011f1-011f 0:R(20)

Table 2 Shifts (m0(N2–CO2, Ne–CO2, and (CO2)2)–m0(CO2)) of m3 band origins of N2–C16O2, Ne– C16O2, and (C16O2)2 (Dm0(16O)) and N2–C18O2, Ne–C18O2, and (C18O2)2 (Dm0(18O)) by complex formation (cm1).

RQ

Dm0(16O) Dm0(18O) Shift ratiof

(J)

0

1

RR

(J)

0

5

10 0

3

a b c

Wavenumber/cm-1 Fig. 1. The RQ0(J) and RR0(J) branches of

d e 14

N2–12C18O2.

f

Ref. [5]. Ref. [13]. Ref. [14]. Ref. [3]. Ref. [4]. Dm0(18O)/Dm0(16O).

N2–CO2

Ne–CO2

(CO2)2

0.48457(9)a 0.5250(1) 1.0834(3)

0.1363(3)b 0.1432(2)d 1.051(3)

1.62884(5)c 1.6236(2)e 0.9968(1)

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T. Konno et al. / Journal of Molecular Spectroscopy 270 (2011) 66–69 Table 3 Changes in intramolecular potential parameters for CO2, DU2 and DU4, by complex formation.

N2–CO2 Ne–CO2b (CO2)2c a b c

DU2(N m1)a

DU4(1021 N m3)

2.220 0.566 1.868

19.1 4.80 8.92

a

DU 2 S2a ð1023 JÞ

DU 4 S4a (1023J)

a

17.98 4.58 15.13

a

12.5 3.15 5.85

See Eq. (1). Ref. [3]. Ref. [4].

3.3. Change in the intramolecular potential caused by complex formation

Table 4 Effective structural parameters, R (Å) and (90°H) (°), and inertial defects (lÅ2) of N2–CO2.

A different approach to examine Dm0 is to ascribe Dm0 to the influence of the N2 molecule on the intramolecular potential of CO2 by complex formation [1]. The interaction between N2 and CO2 causes distortion of the wavefunctions of bonding electrons and hence, induces a direct change in the intramolecular potential, U(Sa), of CO2 [1–4]. Since the modes of CO2 moiety in N2ACO2 should differ from that of isolated CO2 due to the coupling between the CO2 intramolecular modes and the vdW intermolecular modes, the effective potential for N2ACO2 that reproduces its band origin varies with N2. The following treatment based on this model [1– 4,15] is applicable to our analysis of the isotope effect on Dm0. The change, DU(Sa), is calculated with the approximation that it is a quadratic plus quartic function of the internal coordinate reppffiffiffi resenting antisymmetric stretching, Sa ¼ ðDr1  Dr2 Þ= 2, where r1 and r2 denote two CAO distances as described in [1–4],

DUðSa Þ ¼ UðSa Þcomplex  UðSa Þmonomer ¼ DU 2 S2a þ DU 4 S4a

00

R (90°H00 ) D00 b R0 (90°H0 ) D0 b

N2–12C16O2a

3.7285(5) 6.85(3) 2.26(13) 3.7303(5) 6.86(2) 2.33(12)

3.7313(6)c 7.20(2) 2.44(12) 3.7324(4) 7.23(2) 2.354(83)

a

Ref. [5]. D = Ic  Ia  Ib. Using C00 = 1744.00(89) MHz determined by the microwave spectroscopy [7], 00 R is calculated to be 3.7303(12) Å. b c

constants; the effective NcmAC separation, R, and NcmACAO angle, H, are calculated from the expressions [5]

R¼ ð1Þ

First, U2 and U4 values of U(Sa)monomer is determined so as to reproduce m0’s for 12C16O2 and 12C18O2, where m0’s are calculated numerically by use of Johnson’s method [16]. Then, parameters DU2 and DU4 are fitted so as to reproduce m0’s for N2A12C16O2 [5] and N2A12C18O2; they are listed in Table 3 with those for NeACO2 [3] and (CO2)2 [4]. The term values are calculated at the root-meansquare amplitude of Sa of the v = 1 state, 9  1012 m, since the isotope effect on v = 1 is larger than that on v = 0; they are also given in Table 3. All the DU 2 ; DU 4 ; DU 2 S2a , and DU 4 S4a values are the largest for N2ACO2. The positive quadratic term of N2ACO2 as well as NeACO2 [3] is also nearly canceled by the negative quartic term. The origin of this ‘anomaly’ for N2ACO2 as well as NeACO2 in Dm0 (18O)/Dm0 (16O) can be interpreted by a delicate balance of the contributions of the quadratic and quartic potential terms in Eq. (1). The foregoing discussion suggests multiple sources for the shift of the band origin observed for CO2 complexes as viewed from the quadratic and quartic potential terms. The main origin for the shift is the effect caused by the DU2 and DU4 terms; in case of the rare gas–CO2 complexes, the light Ne atom readily oscillates with the vibration of CO2, this effect may originate from the mixing of CO2 antisymmetric mode with the vdW intermolecular modes [3], whereas for N2ACO2 the effect may increase by accompanying any rocking motion of the N2 molecule. Recently, theoretical shifts for rare gas–CO2 complexes are reported to reproduce well the experimental ones by including the CO2 antisymmetric vibration into the ab initio calculations [17,18]. The obtained molecular constants in the present study will provide useful data for such calculations of the N2ACO2 complex.

N2–12C18O2

2

  1=2 k 1 1 1   l C bCO2 bN2

sin H 

bCO2 A

ð2Þ

ð3Þ

where k is the conversion factor 505 379 MHz uÅ2, l the reduced mass of the pseudodiatomic and bCO2 and bN2 are the rotational constants of the CO2 [8,9] and N2 [19] monomers, respectively. The results for the N2A12C18O2 and N2A12C16O2 species are compared in Table 4, where (90°H) represents root mean square of the angle deviation. As discussed in [1–4], the expected shrinkage of the R values and increment of the H values for the 18O species relative to the 16O species are obtained. The inertial defects, D = Ic  Ia  Ib are essentially isotope-invariant. In Eq. (2), a planar structure of N2ACO2 is assumed, while a large-amplitude out-of-plane motion of N2 is reported by the microwave spectroscopy [7]. Since the contribution of the momentum of inertia of N2 in Eq. (2) should be reduced on the existence of the out-of-plane motion, the R values in Table 4 are slightly underestimated. In order to assess the contribution of the out-of-plane motion, we have carried out rough ab initio calculations of N2ACO2. The barriers for the out-of-plane and in-plane motions of N2 are obtained by MP2/6-311+G(3df) calculations [20] to be 192 cm1 and 241 cm1, respectively, which is essentially similar to the results in Ref. [7]. The mean amplitude (HWHM) for the zero-point out-of-plane vibration is estimated to be 20°, which gives an increase of about 6 MHz of C. As a result, the NcmAC separation, R’s in Table 4, become about 0.008 Å larger when the out-of-plane motion of N2 is taken into account. Acknowledgments

3.4. Effective structural parameters The effective (vibrationally averaged) structures of N2ACO2 in the ground and excited states are obtained from the rotational

The authors are grateful to Mr. Shin-ichi Fukuda for his technical assistance and also grateful to Mr. Satoshi Onaya for his help in the ab initio calculations.

T. Konno et al. / Journal of Molecular Spectroscopy 270 (2011) 66–69

Appendix A. Supplementary material Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://library.osu.edu/sites/ msa/jmsa_hp.htm). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jms.2011.09.004.

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