A Study of 2-Iodobutane by Rotational Spectroscopy - ACS Publications

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A Study of 2-iodobutane by Rotational Spectroscopy Eric A. Arsenault, Daniel A Obenchain, Yoon Jeong Choi, Thomas A Blake, Stephen Anthony Cooke, and Stewart E. Novick J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b06938 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 25, 2016

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The Journal of Physical Chemistry

A Study of 2-Iodobutane by Rotational Spectroscopy Eric A. Arsenault,

†

Daniel A. Obenchain,

S. A. Cooke,

¶

†

Yoon Jeong Choi,

and Stewart E. Novick

†

Thomas A. Blake,

‡

∗,†

†Department of Chemistry, Wesleyan University, Hall-Atwater Laboratories, 52 Lawn Ave,

Middletown, CT 06459 ‡Pacic Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99354 ¶School of Natural and Social Sciences, Purchase College SUNY, 735 Anderson Hill Rd,

Purchase, NY 10577 E-mail: [email protected] Phone: 860-685-2679

1

ACS Paragon Plus Environment


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Abstract The rotational transitions belonging to 2-iodobutane (sec-butyl-iodide, CH3 CHICH2 CH3 ) have been measured over the frequency range 5.5-16.5 GHz via jetpulsed Fourier transform microwave (FTMW) spectroscopy. The complete nuclear quadrupole coupling tensor of iodine, χ, has been obtained for the gauche (g)-, anti (a)-, and gauche0 (g0 )-conformers, as well as the four 13 C isotopologues of the gauche species. Rotational constants, centrifugal distortion constants, quadrupole coupling constants, and nuclear spin-rotation constants were determined for each species. Changes in the χ of the iodine nucleus, resulting from conformational and isotopic dierences, will be

discussed. Isotopic substitution of g-2-iodobutane allowed for a rs structure to be determined for the carbon backbone. Additionally, isotopic substitution, in conjunction with an ab initio structure, allowed for a t of various r0 structural parameters belonging to g-2-iodobutane.

Introduction The electrophilic addition of hydrogen halides to alkenes is a fundamental two-step reaction discussed in most introductory organic texts:

1

rst a carbonium ion is formed with the

transfer of the proton from the hydrogen halide to the alkene, and second the halide ion bonds to the carbonium ion.

Where on the carbon backbone the halide ion attaches is

dictated by the relative stability of the carbonium ion formed in the rst step. An ion formed at a tertiary carbon position is more stable than that at a secondary position, which in turn is more stable than an ion formed at a primary carbon position. These relative stabilities lead to Markovnikov's empirical rule explaining the preponderance of one product isomer formed relative to another in hydrogen halide additions to alkenes. The overall energetics of these reactions are further complicated by conformational equilibria.

1,2

Our interest is in

examining the structures of these haloalkane conformers and, eventually, their relationship to the carbonium ion transition state.

2


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Halobutanes represent a suciently complex, yet tractable, conformer system to study using free-jet supersonic cooling and Fourier transform microwave spectroscopy. We begin here by examining the conformers of 2-iodobutane. Iodine, with its large nuclear quadrupole moment and possible spin-rotation coupling, makes the resulting microwave spectrum more complex and challenging to assign and t, but it aords the opportunity of gathering more detailed information about the molecule's electronic wave function parameters.

3

The rst spectroscopic investigation of 2-iodobutane belonged to a larger infrared (IR) study of 2-haloalkanes carried out by Benedetti and Cecchi

4

over 40 years ago.

The IR

spectra for each haloalkane revealed the presence of three conformational isomers, related via a rotation about the C 2 −C3 bond of the four carbon chain.

The conformations of

2-iodobutane, found in this work, agree with the conclusions of Benedetti and Cecchi. Additionally, the

13

C isotopologues of g-2-iodobutane were observed in natural abundance, leading

to a more rigorous determination of the structure of this particular conformation. The nuclear quadrupole coupling constants (NQCCs) belonging to the spectroscopically observed 2-iodobutane species in the present work will be compared with each other and to previously investigated iodoalkanes, in order to gain insight into how substitution aects the electronic environment at the nucleus of the iodine atom.

Experimental Instrumentation The initial rotational spectrum of 2-iodobutane was collected over a frequency range of 7-13 GHz, shown in Figure 1, on a chirp-pulsed Fourier transform microwave (FTMW) spectrometer. This instrument is based on the design by Pate and coworkers outlined in detail elsewhere.

6

5

and has been

The circuitry of this instrument will be described briey. An

8, 10, or 12 GHz microwave center frequency,

ν , is mixed with a 6 µs linear frequency sweep,

from DC to 1 GHz. The successive microwave radiation,

ν ±

3


1 GHz, is amplied and then


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broadcast through a microwave horn antenna into the molecular beam. The radiation then induces a polarization in the coincident supersonic expansion of the gas-phase molecular sample. Following a 1

µs

delay, a second microwave horn antenna collects the free induction

decay (FID). The FID is fast Fourier transformed and directly digitized on a Tektronix TDS6124C Digital Oscilloscope. 800,000 points of the FID are collected over a time period of 20

µs,

where one point is obtained every 25 ps. Measured rotational transitions have an

average line width of 80 kHz with an uncertainty of Transitions for all four

13

±

8 kHz in the center frequency.

C isotopologues, as well as ancillary transitions for all of the con-

formers, were measured with a Balle-Flygare type spectrometer. been formerly described in detail.

8,9

7

This instrument has also

In short, microwave radiation, lasting 0.9

the sample concurrently undergoing supersonic expansion. After a delay of 26 of the polarized sample is collected for 102.4

µs and digitized.

µs, µs,

polarizes the FID

Between a few hundred and a

few thousand averages were collected for each molecular transition, in order to improve the signal-to-noise ratio. The measured transitions have an average line width of 5 kHz with an uncertainty of

±

3 kHz in the center frequency.

The sample was purchased from Sigma-Aldrich ( ≥

98% CH3 CHICH2 CH3 ) and used with-

out further purication. The volatile liquid sample (boiling point 119-120 into a glass U-form tube containing copper beads as a stabilizer. argon (99.999%, Airgas) was bubbled through the sample.

◦

C) was pipetted

One atmosphere of dry

This mixture was then pulsed

through a solenoid valve into the chamber of the spectrometer, which is held at a pressure of

∼ 10−6

Torr. The molecules in the gas pulse undergo supersonic expansion, leaving them

rotationally cold (1-2 K) and in only the lowest energy conformations, but yet, g-, a-, and

0 g -conformers were observed. Sample treatment, as described above, was identical for both instruments.

4


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Figure 1: The experimental rotational spectrum of 2-iodobutane is shown above the baseline and the simulated spectra of the various conformers are shown below.

This portion of

spectrum illustrates the heavy overlap of the hyperne structure due to the iodine nucleus 0 in each of the three conformers. The predicted gauche-, anti-, and gauche -2-iodobutane transitions are represented by purple, red, and green, respectively. intensities have been scaled using the relative

ab initio

Theoretical transition

energies.

Quantum Chemical Calculations Quantum chemical calculations were performed using the GAUSSIAN09 Revision A suite to obtain

ab initio

10

structures for CH 3 CHICH2 CH3 . A coordinate scan at the APFD/321G*

level was rst employed, in order to determine likely ground state molecular geometries. From this scan, multiple low-energy structures were found.

However, only two structures

from this calculation were observed in the spectra, which have been labeled as the g- and

0 g -conformers. The subsequent structures were optimized at the MP2 level of theory using a 6-311G* basis set for the iodine atom, which was imported from the EMSL Basis Set Library

11,12

and a 6-311G++(2d,2p) basis set for the remaining hydrogen and carbon atoms.

An additional calculation for the a-conformer, not obtained in the coordinate scan, was performed at the same level of theory as the other optimizations. Zero point energy (ZPE) corrections were calculated for each optimized structure. Results from the ZPE corrections did not contain any imaginary frequencies, which indicates that these three structures are all true local minima on the potential energy surface. Images of these three

ab initio

structures

are presented in Figure 2 and the results of the calculation are presented in Table 1. The

0 rotational constants, belonging to the a-, g -, and g-conformers, are in basic agreement with

5



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the experimental results.

ab initio

Figure 2: Illustrations of the three structures. The C 1 −C2 −C3 −C4 dihedral angles 0 ◦ ◦ ◦ for the a-, g -, and g-conformers are 64 , -62 , and 171 , respectively.

Spectral Assignments The microwave assignments for the three conformers and all four

13

C isotopologues of gauche-

2-iodobutane were completed with the aid of Pickett's SPFIT/SPCAT

13

software. All of the

conformers were t initially from broadband spectra gathered on the chirp-pulse FTMW instrument, where the AABS package

14

was used jointly with Pickett's programs.

Tables

2 and 3 contain the nal spectroscopic constants for the three conformations observed and for the gauche species and its four

13

C isotopologues, respectively.

there is approximately a 4% dierence between all of the tational constants.

The

ab initio

ab initio

It can be noted that and experimental ro-

NQC tensors of iodine are all quite a bit dierent than

the experimental results. The only agreement is in the trend in relative magnitude of the diagonal and o-diagonal NQCCs. The rather poor prediction of the NQC tensor is due to the fact that a core potential was used to estimate the interaction energies of the many core electrons of iodine. For consistency, the experimentally determined o-diagonal NQCCs are presented with signs in agreement with the

ab initio

values. However, the only certainty that

exists about the signs of the o-diagonal terms is that the product of the three o-diagonal elements must be negative.

15

6


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Table 1:

Ab initio results of 2-iodobutane at the MP2 level of theory 0

Parameters

a

g

g

A (MHz) B (MHz) C (MHz) χaa a (MHz) χbb (MHz) χcc (MHz) χab (MHz) χac (MHz) χbc (MHz) ∆Eb (cm−1 )

6072

3612

4273

1160

1645

1479

1014

1189

1248

-812

-684

-650

395

272

287

417

411

363

-256

-441

-416

-207

-212

-311

-43

-85

-118

166

0

210

174

0

237

64

171

-62

∆EZP E c (cm−1 ) ◦

Dihedral Angle ( )

a NQCCs resulting from the presence of iodine. b Energies relative to lowest energy conformer.

c Energies relative to lowest energy conformer with ZPE corrections.

The g-conformer, an asymmetric top with

κ = −0.62,

contains a dipole moment that

0 projects onto the a-, b-, and c-principal axes. Both the a- and g - species are near prolate, with

κ = −0.94

and

κ = −0.85,

and also contain a dipole moment with three projections

in the principal axis system. Due to the asymmetry and large quadrupolar nuclei present in each species, a rich variety of transition types were observed. Spectral assignments for the

0 a-conformer were based on Q- and R- branch transitions. For the g - species, Q-, R-, and S-branch transitions ( ∆J =+2) were also observed. The t for the g-conformer included P-, Q-, R-, and S-branch transitions. Figure 3 presents an

a -type S-branch transition belonging

to g-2-iodobutane. The assignments for the four gauchewere based primarily on

13

C isotopologues, all present in natural abundance,

a -type R-branch transitions.

A few

b -type R-branch transitions and

a S-branch transition, also included in the spectral ts for each species, helped signicantly to better determine the

A rotational constant.

All four of these ts consist of fteen parameters.

7



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However, six parameters, namely the centrifugal distortion constants, nuclear spin-molecular rotation constants, values.

Page 8 of 24

DK , d1 , and d2 , and the

Caa , Cbb , and Ccc , were held constant to the parent

These terms were unable to be t from the small number of observed transitions

belonging to the lowest energy conformation, the g-

13

C isotopologues.

13

C isotopologues

belonging to the two other conformers were not assigned. The transitions belonging to these species were both lacking in intensity and heavily tangled in a slew of other transitions.

Hyperne Structure The

I =

5 nuclear spin of iodine, results in the presence of hyperne structure in the ro2

tational spectrum of 2-iodobutane. The observed hyperne structure is a result of nuclear quadrupole and spin-rotation coupling. The Hamiltonian that accounts for these complications is of the form:

1618

ˆ=H ˆR + H ˆ CD + H ˆQ + H ˆ SR . H

(1)

ˆ R and H ˆ CD are the Hamiltonian terms accounting for molecular rotation and centrifugal H distortion, respectively. written:

ˆQ H

is the nuclear quadrupole coupling Hamiltonian, which can be

19

ˆQ = H

X 1 χαβ [I , I ] α β + 2I(2I − 1)

(2)

α,β

and then, with some manipulation, this can be written in a form appropriate for use with Pickett's SPFIT/SPCAT:

ˆQ = H

13

1 3 1 1 { χaa [Ia2 − I 2 ] + (χbb − χcc )[I+2 + I−2 ] + χab [Ia Ib + Ib Ia ] 2I(2I − 1) 2 3 4 +χac [Ia Ic + Ic Ia ] + χbc [Ib Ic + Ic Ib ]}

8


(3)

Page 9 of 24

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where the

χij

ˆ SR , H

coupling tensor. be expanded as:

where

Cii

terms correspond to the components of the nuclear electric quadrupole the Hamiltonian that accounts for nuclear spin-rotation coupling can

20

ˆ SR = Caa Ia Ja + Cbb Ib Jb + Ccc Ic Jc H

(4)

are the diagonal nuclear spin-rotation constants.

Rotational transitions are

labeled by quantum numbers of the form

0 0 00 00 JK 0 0 F ← JK 00 K 00 F , where F a Kc a c

is the total angular

momentum quantum number that includes the coupling of spin angular momentum with the rotational angular momentum of the molecule, given by

F = I + J.

Discussion Structural Determination The ve unique sets of rotational constants for g-2-iodobutane, from the parent species and four

13

C isotopologues, allowed for structural determination via isotopic substitution. With

the aid of the STRFIT structural tting program,

21

eight selected geometric parameters were

obtained using these fteen spectroscopic rotational constants, where served as an initial structure.

ab initio

coordinates

Table 4 lists the four bond lengths, two bond angles, and

two dihedral angles that give the structure of the carbon backbone and C −I bond. The C 1 -

◦ C2 -C3 -C4 dihedral angle was determined to be 172.7(16) , thus indicating that the carbon chain is non-planar. The

r0

coordinates obtained from this structural t were used in later

calculations, namely when performing a rotation of the quadrupole tensor into the C −I bond. A Kraitchman analysis

22

yielded

rs

coordinates for the carbon chain, with respect to the

principal axes of g-2-iodobutane. This analysis was performed to serve as a conrmation that the positions of the assigned carbons are correct. A comparison between these experimentally derived coordinates and

ab initio

coordinates is presented in Table 5.

9


It should be noted


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a -type

Page 10 of 24

3 ← 221 21 , of the 2 parent conformer of g-2-iodobutane is shown, where the average of each peak is taken to be Figure 3: The Doppler doublet of the

S-branch transition,

404

the transition frequency, 11011.621 MHz. Labels on the y-axis of the plot were omitted, since the intensity of the transition is arbitrary. The transition was measured on a Balle-Flygare type spectrometer with 100 averages, with a signal-to-noise ratio of 76.

10



11

d

2.85(11) 4.69(12) 3.65(7)

4.0(7) 3.74(22) 4.47(24)

3.8

2.7

212

-176.840(32)

-92.25(7)

102

-456.84(5)

-452.170(28)

0

3.0

72

3.52(27)

4.18(30)

2.7(5)

-227.51(4)

-615.34(6)

-792.17(4)

687.169(12)

569.007(11)

-1256.176(6)

-0.088(4)

4.54(7)

-0.618(18)

0.3546(29)

1287.89606(38)

1520.53128(26)

-85

-212

-441

411

272

-684

1189

1645

3612

g

ab initio

-43

-207

-256

417

395

-812

1014

1160

6072

a

ab initio 0

-118

-311

-416

363

287

-650

1248

1479

4273

g

χbc ,

q P

(obs − calc)2 /Nlines

must be negative. These terms are presented with signs in agreement with the

d Number of transitions used in the t. e Root mean square deviation of the t,

and

results.

b NQCCs resulting from the presence of iodine. c The relative signs of the o-diagonal NQCCs can not be determined. It is only known that the product of the three,

in units of the least signicant gure.

g 4433.8638(7)

a Numbers in parentheses give standard errors (1 σ , 67% condence level)

e RMS (kHz)

N

-822.076(20)

747.154(14)

771.233(17) -497.892(35)

582.497(14)

779.401(12)

-0.1256(10)

-0.0141(11)

-1329.651(2)

1.034(7)

-6.12(25)

-1550.634(13)

-0.01970(35)

0.3488(14)

0.0908(16)

1231.19182(7)

3726.51649(11)

1049.42041(11)

a 1706.60968(12)

6276.1041(8)

A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) χaa b (MHz) χbb (MHz) χcc (MHz) χab c (MHz) χac (MHz) χbc (MHz) Caa (kHz) Cbb (kHz) Ccc (kHz)

g

1200.61256(18)

a

Parameters

Experimental

χab , χac ,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2: Spectroscopic parameters of three conformations of 2-iodobutane

Page 11 of 24 The Journal of Physical Chemistry


12

f (kHz) 2.7

C1

0.8

39

[3.65]

[4.69]

[2.85]

-169.526(30)

-460.40(31)

-789.72(13)

746.833(19)

609.264(14)

-1356.097(13)

[-0.0197]

1.7

58

[3.65]

[4.69]

[2.85]

-173.04(17)

-452.34(31)

-814.05(12)

749.494(13)

589.897(11)

-1339.391(6)

[-0.0197]

[-0.1256]

[1.034]

[-0.1256]

0.3441(20)

g [1.034]

1225.72244(13)

1697.83972(12)

C3

ab initio

1.6

38

[3.65]

[4.69]

[2.85]

-180.07(44)

-461.4(7)

-826.10(33)

744.672(36)

578.842(27)

-1323.515(23)

[-0.0197]

[-0.1256]

[1.034]

0.3385(32)

1216.26864(20)

1677.48112(18)

3722.6838(8)

13 C4

1.2

40

[3.65]

[4.69]

[2.85]

-187.00(28)

-451.6(5)

-864.80(20)

747.476(27)

544.173(20)

χbc ,

obtained for parent.

g Numbers in square brackets indicate values held constant to those

q P (obs − calc)2 /Nlines

must be negative. These terms are presented with signs in agreement with the

e Number of transitions used in the t. f Root mean square deviation of the t,

and

results.

χab , χac ,

-1291.649(17)

[-0.0197]

[-0.1256]

[1.034]

0.3478(24)

1207.29040(14)

1674.57492(14)

3659.4255(8)

13

c NQCCs resulting from the presence of iodine. d The relative signs of the o-diagonal NQCCs can not be determined. It is only known that the product of the three,

in units of the least signicant gure.

C2

3710.83914(30)

13

0.3386(15)

1213.32896(9)

1697.90412(9)

3606.32964(26)

13

a From MP2 level calculation. b Numbers in parentheses give standard errors (1 σ , 67% condence level)

RMS

212

-456.84(5)

-212

-822.076(20)

-441

3.65(7)

747.154(14)

411

582.497(14)

272

e N

-1329.651(2)

-684

4.69(12)

-0.01970(35)

-0.1256(10)

2.85(11)

1.034(7)

0.3488(14)

-176.840(32)

1231.19182(7)

1189

-85

1706.60968(12)

1645

b

3726.51649(11)

A (MHz) B (MHz) C (MHz) DJ (kHz) DK (kHz) d1 (kHz) d2 (kHz) χaa c (MHz) χbb (MHz) χcc (MHz) χab d (MHz) χac (MHz) χbc (MHz) Caa (kHz) Cbb (kHz) Ccc (kHz) 3612

g-Parent

Prediction

Parameters

a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Table 3: Spectroscopic parameters for g-2-iodobutane

The Journal of Physical Chemistry Page 12 of 24

Page 13 of 24

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that the c-coordinate for C 1 is an imaginary number. Although this coordinate is included in Table 5, it is best to assume that it is simply near zero. Table 5 also oers a comparison between the Kraitchman analysis and

ab initio

results, which are in good agreement.

Nuclear Quadrupole Coupling Tensor of Iodine Upon inspection of Tables 2 and 3, it is immediately obvious that the elements of the NQC tensor are quite dierent for each species of 2-iodobutane. These dierences are a result of the dierent orientations of the principal inertial axes in the conformations and isotopologues with respect to the C −I bond. In order to make a meaningful comparison of these tensors, they should all be expressed in individual coordinate systems that are not dependent upon the conformations or the isotopic variations. One such set of frames are those in which the

χ

tensors themselves are diagonalized. Utilizing Kisiel's program, QDIAG,

NQC tensor of iodine was diagonalized for each species of 2-iodobutane.

27

the complete

Diagonalization

transforms the NQC tensor from the inertial axis system of the molecule to the principal axis system of the quadrupolar nucleus, iodine, in this case. A comparison of the diagonalized quadrupole tensor,

χ,

for iodine in each of the three

observed conformations is presented in Table 6. Comparing values of

ηχ,

which is a measure

of the asymmetry of the tensor, in Table 6 reveals subtle changes in the electronic nature of the C−I bond, or more specically, changes in the electric eld gradient at the nucleus of the iodine atom, due to conformational dierences. A NQC tensor with an indicate a cylindrically symmetric tensor. Values of

ηχ

ηχ

of 0 would

0 for the a-, g-, and g -conformers were

0.014513(16), 0.026268(20), and -0.00242(4), respectively. This parameter indicates that the

0 g -conformer is 6 times more symmetric than the a-conformer, which is just under twice as symmetric as the g-conformer. Interestingly, the order in increasing symmetry between for the three conformers follows the trend in the increasing relative

χ

ab initio energies between

the three conformers. The aect of geometric changes on the NQC tensor can also be seen upon a comparison of

χzz

between the three conformers. After comparing the nal values

13



14

1.497(6) 1.548(6) 2.166(4)

C2 -C3 C3 -C4 I-C2

114.17(23)

-63.85(31)

(I-C2 -C3 -C4 )

σ

0.018

Structural Fit Error χ2 0.0024

172.7(16)

(C1 -C2 -C3 -C4 )

◦ Dihedral ( )

112.9(10)

6 (C2 -C3 -C4 )

a

6 (C1 -C2 -C3 )

◦ Bond Angle ( )

1.536(12)

C1 -C2

Bond Length (Å)

structural parameters of gauche-2-iodobutane

a Numbers in parentheses give standard errors (1 σ , 67% condence level) in units of the least signicant gure.

r0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Table 4: STRFIT

The Journal of Physical Chemistry Page 14 of 24

2.1389(7) 0.6646(23) 0.132(11)

1.1801(13) 2.2422(7) 2.3616(6)

C1

C2

C3

C4

0.113(13)

0.354(4)

0.370(4)

0.052(29) i

|c|

2.4

2.3

1.2

1.2

a

ab initio

-1.6

-0.15

0.67

2.2

b

0.13

-0.36

0.37

-0.023

c

Gauche Coordinates

a Values in parentheses give absolute Costain errors of the least signicant gure.

1.6019(9)

|b|

a 1.2170(12)

Kraitchman Coordinates |a|

Atoms

ab initio coordinates for gauche-2-iodobutane

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 5: Kraitchman versus

Page 15 of 24 The Journal of Physical Chemistry


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

Table 6: Conformational comparison of the diagonalized NQC tensor of iodine in 2-iodobutane Parameters

χzz χyy χxx

a

a

0

g

g

-1731.291(23)

-1730.09(4)

(MHz)

-1737.458(21)

(MHz)

881.337(18)

888.384(20)

862.95(4)

(MHz)

856.121(20)

842.907(28)

867.14(5)

0.014513(16)

0.026268(20)

-0.00242(4)

ηχ

b


bη

χ is a measure of the asymmetry of the nuclear quadrupole coupling tensor, where yy ηχ = χxxχ−χ . zz

Table 7: Comparison of the diagonalized NQC tensor of iodine 2-iodobutane with other iodoalkanes

χzz

Molecule

(MHz)

Reference

23

CH3 I

-1934.080(10)

Wlodarczak et al.

CH3 CH2 I

-1815.693(210)

Boucher et al.

trans-CH3 CH2 CH2 I

-1814.55(55)

Fujitake and Hayashi

gauche-CH3 CH2 CH2 I

-1805.16(56)

Fujitake and Hayashi

CH3 CHICH3

-1741.47(75)

Ikeda et al.

a−CH3 CH2 CHICH3

-1737.458(21)

This work

g−CH3 CH2 CHICH3

-1731.291(33)

This work

-1730.09(4)

This work

0

g -CH3 CH2 CHICH3

16


24

26

25 25

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 8: Rotation of the diagonalized NQC tensor of iodine into the principal axis system of g-2-iodobutane Parameters

χzz χyy χxx

(MHz)

-1731.291(23)

(MHz) (MHz)

ηχ b

13

g-Parent

a

13

C1

C2

13

C3

13

C4

-1731.21(14)

-1731.49(13)

-1731.24(34)

-1731.25(23)

888.384(20)

888.34(8)

888.61(9)

888.60(24)

888.51(16)

842.907(28)

842.88(14)

842.89(14)

842.63(35)

842.74(25)

0.026268(20)

0.02626(9)

0.02640(10)

0.02655(25)

0.02644(17)


bη

χ is a measure of the asymmetry of the nuclear quadrupole coupling tensor, where χxx −χyy . χzz

ηχ =

ab initio NQC tensor due to isotopic substitution

Table 9: Changes in the

Parent

χzz χyy χxx

13

C Bond Length Correction

(MHz)

-896.4

-896.7

(MHz)

444.5

444.7

(MHz)

451.9

452.1

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of

χzz

Page 18 of 24

for each conformer, at most, only a 0.4% dierence was observed.

powerful conclusions can be drawn from the values of

χzz

However, more

obtained for 2-iodobutane upon

contrasting this work with a series of previous studies on iodoalkanes. Table 7 presents a selection of such work. There is a notable trend, namely that the magnitude of with increasing carbon substitution in the iodoalkane.

χzz

decreases

This change in magnitude is more

pronounced when the degree of substitution increases on the carbon directly bonded to the iodine atom. More simply put, | χzz | is less for an iodine atom bonded to a secondary carbon than it is for an iodine atom bonded to a primary carbon. This diagonalization is only possible because the complete tensor was determined, which, in turn, was only possible because the iodine

χ

was large enough that even the o-diagonal terms had spectroscopic

consequences. There is only a 0.23% dierence between

χzz

of a-2-iodobutane and isopropyl

iodide, which is just under three times less than the dierence between

χzz

0 of g -2-iodobutane

and isopropyl iodide, 0.65%. This factor of three can be rationalized by comparing geometric

0 dierences between these species. The g -conformer simply diers more from isopropyl iodide geometrically than the a-conformer does. These comparisons serve quite nicely to show the sensitivity of the nuclear quadrupole of the iodine atom and its ability to serve as a probe of subtle chemically relevant dierences. However, even more subtle changes in the NQC tensor of iodine can be noticed by comparing

χ of the parent species of g-2-iodobutane with its four

seen in Table 8. A comparison of

ηχ

13

C isotopologues. This can be

between these ve isotopic species reveals agreement, at

least within range of their respective errors.

13

C isotopic substitution seems to have no no-

ticeable aect on the asymmetry of the NQC tensor of iodine in g-2-iodobutane. Although there may be no aect in this regard, the values of

χzz

seem to suggest a change in the

projection of the NQC tensor along the z-axis of the iodine nucleus when the directly bonded to iodine is substituted with a in

χzz

between the parent and

13

13

12

C nucleus

C nucleus. When comparing the dierence

C2 isotopologue, the change in

χzz

is 200 kHz. This is just

over a factor of two greater than the largest change observed for any of the other isotopo-

18


Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


logues, where the values of

χzz

between the parent and

13

C1 ,

13

C3 , and

13

C4 isotopologues

only vary by 40 to 80 kHz. Multiple causes for this discrepancy were investigated in order to rationalize this very small dierence. First, the presence of additional nuclear spin-rotation interactions, due to the the magnetic moment of the

13

C nucleus, were investigated. However, the inclusion of these terms in

the Hamiltonian of this isotopologue both did not t or counterbalance the change in the

13

χzz

of

C2 isotopologue. Further evidence against the presence of this interaction was provided

by the fact that the other three isotopic species yielded values of

χzz

nearly identical to the

parent value without the addition of these terms in their respective Hamiltonians. Second, an additional spin-spin interaction between present. No change in

χzz

of the

13

13

C (I

=

1 127 ) and I (I 2

=

5 ) may be 2

C2 isotopologue ensued as a result of including this term

in the Hamiltonian. The term was deemed negligible, as it did not t well and was only on the order of 2 kHz. After a missing term in the Hamiltonian of the cause for this change, the discrepancy in

χzz ,

13

C2 isotopologue was eliminated as a

between the parent species and this isotopo-

logue, can be attributed to the most obvious eect, the increased mass of the carbon atom directly bonded to the iodine.

Upon

13

C isotopic substitution, the vibrationally averaged

C−I bond length should decrease. The fact that the z-axis of the diagonalized nuclear electronic quadrupole tensor is under 2 an observation possible. A further

◦

from the C−I bond axis certainly helped to make such

ab initio

study was performed, to determine the normal

mode vibration frequencies of g-2-iodobutane. The bond length was scaled using the normal mode frequency that is closest to the pure C −I stretch, 599 cm that the reduced mass of the stretch was the reduced mass of

−1

12

, and the approximation

C

− 127 I

decrease in bond length of 1.8 mÅ was found, which was used to scale the

or

13

13

C

− 127 I.

C

− 127 I

A

bond

length. An energy calculation was then performed on this slightly altered structure, at the same level of theory as all previous quantum chemical calculations, to predict the change in the NQCCs. Table 9 presents

χ for each of the two species in question with more signicant

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

gures than are typically presented because the comparison being made is only among

ab

initio

of

values, so the errors in the magnitudes cancels out to rst order. A decrease in

χzz

300 kHz was determined for a bond length decrease of 1.8 mÅ. Thus our measured decrease of 199(13) kHz suggests a C −I bond length decrease of approximately 1.2 mÅ upon

13

C

isotopic substitution.

Conclusion Utilizing high resolution rotational spectroscopy, an intensive investigation of three conformers and four

13

C isotopologues of 2-iodobutane allowed for many chemically relevant

parameters of this haloalkane to be determined.

The observed hyperne structure led to

the complete determination of the NQC tensor of iodine in seven dierent species of 2iodobutane. In this way, iodine served as a probe of the subtle dierences in these species, resulting from both isotopic and geometric dierences.

Acknowledgement The authors thank Wallace (Pete) Pringle for many useful discussions. The cluster at Wesleyan University is supported by the NSF under CNS-0619508. The Pacic Northwest National Laboratory is operated for the United States Department of Energy by the Battelle Memorial Institute under contract DE-AC05-76RLO 1830.

Supporting Information Available 0 13 Final t outputs for the a-, g-, and g -parent species, in addition to the four C isotopologues 0 of the g -conformer, are provided. This material is available free of charge via the Internet at

20


http://pubs.acs.org/ .

Page 21 of 24

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A Study of 2-Iodobutane by Rotational Spectroscopy - ACS Publications

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