Observation of the high-resolution spectrum of the N–H bending vibration of ketenimine CH2CNH

Journal of Molecular Spectroscopy 264 (2010) 100–104

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Observation of the high-resolution spectrum of the N–H bending vibration of ketenimine CH2CNH Fumiyuki Ito ⇑, Taisuke Nakanaga National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan

a r t i c l e

i n f o

Article history: Received 2 September 2010 In revised form 21 September 2010 Available online 29 September 2010 Keywords: Ketenimine FTIR Perturbations Coriolis coupling Large amplitude motion

a b s t r a c t A high-resolution infrared spectrum of ketenimine CH2CNH has been measured in the N–H bending region around 1000 cm1 using an FTIR spectrometer. A rotational analysis of the spectrum revealed perturbations in the vibrational excited state, and they were analyzed in terms of Coriolis coupling and large amplitude motion. The band center of this band has been determined to be 1000.23786(71) cm1. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Ketenimine, CH2@C@NH, is an unstable molecule produced in pyrolytic reactions of various nitriles, and has attracted much attention as a reactive intermediate in organic chemistry [1] and astrochemistry [2,3]. Its isomerization to acetonitrile CH3CN, i.e. most stable C2H3N species, has been experimentally [4] and theoretically investigated [5]. In spite of the interest in such wide areas, spectroscopic information on ketenimine is still limited. The first observation of its infrared spectrum has been done in an Ar matrix [6], followed by microwave spectroscopy by Rodler et al. [7,8]. The inversion splitting associated with the N–H bending motion was measured to be 2.2  106 cm1 (66 kHz) in vibrational ground state, thereby inversion potential function was determined with the use of semi-rigid-bender calculation [8]. Rodler et al. thus showed that ketenimine is the first imine molecule that exhibits inversion splitting observable experimentally. In 1990, we reported the high-resolution IR spectrum of the C@C@N stretching band and its rotational analysis [9]. The present study reports on the subsequent infrared study of this molecule. As listed in Table 1, ketenimine has 12 vibrational modes and the C@C@N stretching mode (previously denoted as m4 and should be renewed as m3) has the largest infrared intensity. Our next target was the second strongest band which involves the N–H in-plane bending. Hereafter we call ⇑ Corresponding author. Address: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba-West, Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan. Fax: +81 29 861 8252. E-mail address: [email protected] (F. Ito). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.09.012

this mode as m6, in accordance with the Herzberg notation [10]. Since this mode is directly correlated to the N–H inversion motion, it would be interesting to see how the m6 mode is affected by the large amplitude motion. 2. Experimental The experimental procedure was already described [9]. In brief, the pyrolytic product of ethylenecyanohydrine (HOCH2CH2CN) in a quartz tube heated at ca. 1050 °C was flowed into a White-type long path cell (l = 10 m). The cell was evacuated by a roots pump (Edwards EH250) backed up by a rotary pump (Edwards E2M40), and the pressure inside the cell was maintained at ca. 80 mTorr to minimize the loss of the reactive species. The spectrum of the m6 band was measured by using a BOMEM DA 3.36 spectrometer equipped with a KBr beam splitter and a liq. N2-cooled HgCdTe detector. The spectrum was taken in the region of 900–1100 cm1, and 16 scans were accumulated with an apodized resolution 0.005 cm1. An overview of the spectrum is shown in Fig. 1. A density functional theory (DFT) calculation was performed to obtain the equilibrium geometry and vibrational frequencies of ketenimine. In view of the previous theoretical studies [1,4,11,12], the DFT calculation was done at the B3LYP/6-311++G(3df,3pd) level. In order to obtain parameters of the bending potential, we also carried out calculations of a transition state and intrinsic reaction coordinates (IRC) at the same level of theory. All calculations were done by using Gaussian 03 [13]. The harmonic and the anharmonic wavenumbers for each normal mode are listed in Table 1, along with the observed vibrational frequency. We can see that the anharmonicity of each mode is relatively small, and that the anharmonic

101

F. Ito, T. Nakanaga / Journal of Molecular Spectroscopy 264 (2010) 100–104 Table 1 Observed and calculated frequency of each normal mode of keteniminea. Previous notationb

Herzberg notation 0

1(A ) 2 3 4 5 6 7 8 9(A0 0 ) 10 11 12

Obs. (Ar matrix)c

1 3 4 5 6 7 10 11 2 8 9 12

Calc.f

Obs. (gas phase)

3479 [3300] (22) 3164 [3040] (9) 2115 [2074] (437) 1439 [1442] (5) 1166 [1149] (18) 1024 [984] (210) 717 [698] (88) 484 [487] (22) 3251 [3110] (1) 1001 [984] (0.007) 896 [868] (53) 422 [425] (0.4)

d

2040

2043.5775

1124 1000 690

1000.2379e

872

a

In cm1. Ref. [9]. c Ref. [6]. d Ref. [9]. e Present study. f Harmonic wavenumbers obtained at the B3LYP/6-311++G(3df,3pd) level. Numbers in square brackets are anharmonic wavenumbers and those in parentheses are infrared intensities in km mol1. b

1.4 1.2

Absorbance

1.0

~0.6 cm-1

0.8 0.6 0.4

1.4

0.2 0.0 1000

1.2

1001

1002

1003

1004

1005

Wavenumber (cm-1)

Absorbance

1.0 0.8 0.6 0.4 0.2 0.0 970

980

990

1000

1010

1020

1030

1040

Wavenumber (cm-1) Fig. 1. An overview of the m6 band. The inset shows an expanded view in the 1000–1005 cm1 region, indicating sequences with interval of ca. 0.6 cm1.

wavenumbers are in better agreement with observed band positions than the harmonic values. 3. Assignment and analysis The shape of the m6 band was, as shown in Fig. 1, not typical for a parallel- nor a perpendicular band of a near-prolate asymmetrictop molecule. A closer look at the spectrum, however, revealed sequences with intervals of ca. 0.64 cm1 (=B + C), as shown in the inset. Moreover, the DFT calculation predicted |lb(m6)/ la(m6)| = 0.196, where la(m6) and lb(m6) denote transition moment of the m6 mode along a- and b-axis, respectively. It was therefore reasonable to conclude that the m6 band is of a-type and to assign parallel transitions for each Kp sub-bands (DKp = 0, ee M eo, oe M oo). By using ground state combination differences (GSCD) and a homemade program to display the Loomis–Wood diagram, each subband for Kp was assigned up to Kp = 7 and J = 35 (total number of assigned transitions = 542). At J00 > 16, we could resolve the K-

type doubling for Kp = 2. A part of the assignments is schematically drawn in Fig. 2. These transitions in each subband were fitted to a simple quartic formula, m = C0 + C1 m + C2 m2 + C3 m3 + C4 m4 (m = -J for P-branch, J + 1 for R-branch), to derive effective molecular constants C0  C4. These effective constants are collected in Table 2. A plot of the subband origin C0 versus K 2p (Fig. 3) gave band center = 1000.20(13) cm1 and DA = aA6 = 0.668(16) cm1. The obtained band center for the m6 band is in good agreement with the result of matrix-isolation spectroscopy (1000 cm1 [6]). On the other hand, the value of aA6 seemed too large; it is much larger than the value of aA3 (0.0891 cm1) [9]. An unusual band shape of the m6 band in Fig. 1 originates from this large vibration–rotation interaction parameter, and it indicates the presence of strong perturbations in the vibrational excited state. It would be also necessary to estimate the effect of the large amplitude motion on the aA6 , since the value of A of ketenimine increases more than 40% from the Cs equilibrium geometry (Ae (calc.) = 6.8327 cm1) to the C2v saddle point (Ae (calc.) = 9.6013 cm1).

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F. Ito, T. Nakanaga / Journal of Molecular Spectroscopy 264 (2010) 100–104 Table 3 Contributions of various factors to the vibration–rotation interaction constant aA6 obtained from DFT calculationa and rigid bender calculation. Contribution

Values

aA6 (harmonic) aA6 (Cor)

0.0335 0.5968 From m10 0.2172 m11 0.3394 0.1278

aA6 (anharm) aA6 (total) aA6 (large amplitude)b aA6 (obs.) a b

0.75816 0.20 0.6684

Obtained at the B3LYP/6-311++G(3df,3pd) level. Defined as h0+|laa|0+i  h1|laa|1i. See text for detail.

1025

1020

C0 (cm-1)

Fig. 2. The Loomis–Wood diagram of the m6 band in the region of 1001.28– 1006.4 cm1. Each subband for Kp is shown as a broken lines.

4. Results and discussion

1015

1010

For these two effects, the following assessments can be made quantitatively.

1005

4.1. Perturbations due to nearby states

1000

Table 3 lists the aA6 obtained from the DFT calculation and contributions from various factors. In the second order perturbation treatment of semi-rigid molecules, a consists of three terms [14]:

0

30

40

50

Fig. 3. A plot of subband origins (C0) versus K 2p . The solid line was determined by least-squares fitting.

where a(harmonic) is the direct harmonic contribution, a(Cor) the second-order Coriolis contribution, and a(anharm) is the contribution from cubic anharmonicity Each contribution was obtained from the DFT calculation. For a(Cor), the contribution from each of nearby states was evaluated from the following formula:

2A2 X A 2 3x26 þ x2n ð1 Þ : x6 n–6 6n x26  x2n

20

Kp2

aðtotalÞ ¼ aðharmonicÞ þ aðCorÞ þ aðanharmÞ;

aA6 ðCorÞ ¼ 

10

next. This term originates from off-diagonal anharmonicity and should be treated separately from the large amplitude effect where diagonal terms play roles. The sum of values of a(Cor)’s from m10 and m11 is almost comparable to the observed aA6 , indicating that the m6 state is seriously perturbed by these two states. In fact, we could observe a sign of local level crossing at J0 = 7, K0p = 5, as shown in Fig. 4. In this case, the counterpart should be m10 band, since it is accidentally degenerate with the m6 band. Although we tried to assign the sequence of the m10 band, it was not successful so far. The main reason is that the m10 band is nearly infrared-inactive (0.007 km mol1), and that it is very weak even with borrowed

ð1Þ

We can see that the Coriolis coupling is dominant in a(total) among which m10 (1001 cm1) and m11 (896 cm1) perturb m6 state most effectively. The contribution of cubic anharmonicity comes

Table 2 Effective molecular constants for the A-type sub-bands (DKp = 0) of the m6 banda.

a

Kp

C0

C1

C2

C3

C4

0 1(J1J) 1(J1J–1) 2(J2J–2) 2(J2J–1) 3 4 5 6 7

1000.23597(24) 1000.90919(25) 1000.90734(54) 1002.85988(40) 1002.85873(45) 1005.92105(26) 1009.90898(41) 1014.68588(66) 1020.38473(34) 1024.89309(66)

0.638719(17) 0.635730(15) 0.641604(39) 0.638344(28) 0.638656(31) 0.638495(18) 0.638404(33) 0.638072(30) 0.637996(14) 0.637778(28)

5.069(13)  104 7.611(12)  104 1.380(39)  104 4.890(26)  104 4.898(30)  104 4.144(16)  104 3.959(22)  104 2.977(30)  104 3.711(14)  104 3.422(28)  104

1.054(24)  106 4.93(18)  107 1.010(72)  106 1.15(62)  107 5.23(69)  107 3.38(37)  107 4.72(78)  107 1.46(43)  107 3.69(18)  107 4.46(49)  107

7.34(13)  108 3.49(10)  108 1.369(51)  107 7.62(36)  108 1.00(40)  108 1.58(21)  108 1.33(31)  108 6.20(28)  108 9.1(12)  109 1.61(27)  108

In cm1. Numbers in parentheses are one standard deviation of the least-squares fitting.

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F. Ito, T. Nakanaga / Journal of Molecular Spectroscopy 264 (2010) 100–104

0.010

-8

+7

5000

0.008

(4+2) potential

0.006

DFT potential along IRC

4000

Energy (cm-1)

o-c(cm -1)

0.004 0.002 0.000 -0.002 -0.004

3000

2000

1000

-0.006 -0.008 -0.010 -40

0 -30

-20

-10

0

m

10

20

30

40

Fig. 4. A plot of residual of the fitting versus m (J0 0 for P-branch, J0 0 + 1 for Rbranch) in the subband for Kp = 5. Divergent features at m = 8 and 7 are indicative of perturbation at J0 = 7.

intensity from the m6 mode. Another perturber m11 band is much stronger (53 km mol1), and some of the Q-branch head for a perpendicular band could be observed. But, rotational assignment was not possible due to the poor signal-to-noise ratio. In summary, we could not carry out deperturbation of the m6 band. We did not estimate the effects of combination and overtone bands, but they are higher-order effects and contributions from them would be marginal. 4.2. The effect of the large amplitude motion For simplicity, we employed a rigid-bender model [15,16] to estimate this effect. Parameters for the bending potential were obtained from the DFT calculations. These parameters are in excellent agreement with those derived experimentally [8] as follows:

0.0

0.2

0.4

ρ/ρe

0.6

0.8

1.0

Fig. 6. A comparison of the potential function determined in Ref. [8] (closed circle) and that from the DFT calculation (open circle). The calculated potential was obtained along intrinsic reaction coordinate.

barrier height: HNC angle:

obs. 4700 ± 200 cm1 115.4 ± 0.6°

calc. 4698 cm1 116.9°

The calculated potential curve along IRC also reproduces the observed one, as shown in Fig. 6. In order to write down 4  4 l tensor analytically, we approximated the structure of ketenimine as shown in Fig. 5. We assumed a quartic potential with a quadratic hump as in Ref. [8], and numerical integration of the rigid-bender equation was done using fourth-order Runge–Kutta method. The results of the calculation is summarized in Table 4. The inversion splitting of ketenimine in ground state was predicted to be 7  107 cm1, which is one third of the observed value [8]. It is not surprising, since the inversion splitting obtained from rigidbender calculations is critically dependent on the parameters, as reported previously [8]. Rather, it reproduces the previous rigid bender calculation. The value of the splitting 1 – 0+ is in good agreement with the observed band center of the m6 mode (1000.20 cm1). The predicted inversion splitting at m6 = 1 (2  104 cm1) indicates that the splitting in the m6 band cannot be resolved, consistently with our experiments. The effect of the large amplitude motion is, however, observable in the m6 band as a change of rotational constant A,

DA ¼ aA6 ¼ h1 jlaa j1 i  h0þ jlaa j0þ i ¼ 0:20 cm1 :

ð2Þ

This contribution is smaller than the a(Cor), but not negligible. These estimations indicate that the effect of Coriolis coupling with two nearby states and the effect of large amplitude motion are of the same order. Since we are not sure if these two effects are additive or not, it is reasonable to treat them simultaneously. This is a very challenging and unamenable task, since such non-rigid bender treatment has not been formulated for 6-atomic molecules like ketenimine [17,18]. Another complication is that, there Table 4 Results of the rigid bender calculation of keteniminea. Barrier height (cm1) HNC angle (deg.) Inversion splitting (0 – 0+) (cm1) Inversion splitting (1 – 1+) (cm1) 1 – 0+ (cm1) DAb (cm1) Fig. 5. A geomtry of ketenimine used in the rigid bender calculation. The C@C@N angle was constrained to 180° and CH2@C@N moiety was assumed to be coplaner in order to simplify the analytical expression of 4  4 l tensor. Other structural parameters were taken from the DFT calculation.

4698 116.9 7  107 2  104 993.9 0.20

a Barrier height and HNC angle were taken from the DFT calculation. See text for detail. b DA„h1|laa|1i  h0+|laa|0+i.

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Table 5 Molecular constants of ketenimine in the A-reduce form (cm1)a.

m6 = 1 m0 A B C DJ DJK DK dJ dK

1000.23786(71) 7.39658(27) 0.322145(20) 0.316987(21) 6(10)  108 2.18(13)  105 5.743(25)  103 2.1  109(fixed) 1.7  104 (fixed)

Ground stateb 6.719447 0.322362 0.315856 1.1  107 7.84  106 2.754  104 2.1  109 1.7  105

m6 = 1 calcc 1024 [984] 7.5794e 0.32389 0.31851

Ground state calcc d

6.8212 0.32413 0.31755 9.34  108 8.13  106 3.78  104 2.07  109 4.90  107

a

Numbers in parentheses are one standard deviation of the least-squares fitting. Fixed to those in Ref. [9]. c Obtained from theoretical calculation at the B3LYP/6-311++G(3df,3pd) level. d Harmonic wavenumber. The number in square brackets is anharmonic wavenumber. e Including the effect of Coriolis interactions through perturbation treatment. b

are five vibrational levels below m6 mode, which makes it difficult to separate the N–H bending mode adiabatically. Therefore, we must confess that further analysis to obtain deperturbed molecular constants was not possible in the present stage. Instead, we would like to show the molecular constants of ketenimine in the A-reduced form [19] in Table 5. The large values of A and DK clearly indicate that they are not free from perturbations. 5. Conclusion We measured a high-resolution infrared spectrum of the N–H bending vibration of ketenimine, and analyzed its rotational structure. We found that the molecular constants in vibrational excited state show anomalies due to Coriolis couplings with two nearby states and large amplitude motion. For obtaining deperturbed constants, it would be necessary to utilize non-perturbational treatment of the three relevant vibrational levels in combination with theoretical model to describe large amplitude motion of the N–H bending mode, which is yet unavailable.

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