BERNARD R,ICE .AND HENRY8. U P H I n . 4
G5O
T'ol. 59
R,YhiBN SPECTRA O F SOME ETHER-BORINE ADDITION COMPLEXES BY BERNARD RICE AND HENRYs. UCHID.4l Contribution from the Department of Chemistry, St. Louis University, St. Louis, Missouri Received February 14, 1966
Raman spectra were obtained for the liquid systems, diborane-tetrahydrofuran, diborane-dimethyl ether and diboranediethyl ether. The results were in accord with the formation of an RzO: BH3 addition complex in ench system. Tetrahydrofuran-borine was the most stable and diethyl ether-borine the least stable of these complexes. The Rainan frequencies were assigned to the BHIX skeletal model, where X represented an ether. Potential constants were calciilsted for this model of tetrahydrofuran-borine.
Introduction The present Raman spectroscopic investigation is a study of the addition complex formed when diborane is dissolved in tetrahydrofuran, dimethyl ether or diethyl ether. Evidence for complex formation in these systems has appeared in the literature.2 The previous investigators demonstrated, a t best, that the complex is formed in a ratio of one mole diborane to two moles of ether. However, from a knowledge of the chemistry of the reactants it seemed reasonable to most investigators to imply that the reaction can be represented by the general equation 2Rz0
+ BzHs
2Rz0: BHI
Our Raman spectral measurements justify this assumption. The spectral data permit a comparison of the stability of the three ether-borine addition complexes and the determination of force constants for tetrahydrofuran-borine. Experimental Apparatus and Method.-A three prism spectrograph (manufactured by Lane-Wells), with an averaje dispersion of 22 W./mm. in the range from 4358 to 4916 A., was used. The diborane-ether solutions were maintained in a closed system and low temperature Raman exposures were obtained by the method described by Lord and N i e l ~ e n . ~ Chemicals.-The diborane was prepared by the reaction of lithium borohydride with boron trifluoride. The sample, purified by repeated fractionation, had a vapor pressure of 225 mm. a t -112" in agreement with the literature value. The tetrahydrofuran, diethyl ether and dimethyl ether were dried and distilled in vacuo. The vapor pressure of the purified ethers agreed with the literature values. Solutions.-The Raman spectrum of the diborane-dimethyl ether solution was observed a t two temperatures. The diborane-diethyl ether system was treated in a similar fashion. On the other hand, Raman spectra were obtained a t the same temperature for two diborane-tetrahydrofuran
solutions which differed in concentration. The concentrations of the solutions and the temperatures a t which t,hey were studied are listed in Table I. Data.-In Table 11, the Raman frequencies of the most concentrated of the two diborane-tetrahydrofuran solutions studied are listed. For purposes of comparison, the Raman frequencies of pure tetrahydrofuran are also included in the table. The spectrum of the more dilute diborane-tetrahydrofuran solution differs only in the relative intensity of mme of the Raman linea.
TABLE I1 RARIAN FREQUENCIES O F A DIBORANE-TETRAHYDROFURAN SOLUTION A N D PURETETRAHYDROFURAN Diborane-Tetrahydrofuran Frequency (cm. -1) Intensity
Tetrahvdrofuran Frequency (cni. -1) Intensity
472 3 859 7d 917 8 911 2 1028 076 1049 5d 1068 1170 5 1173 1232 4d 1236 1364 13G4 1145 1450 6 1487 5d 1487 2097" 2 2261 2284 8 2308 0 2523" 2717 1 2874 8d 28G0 2908 0 2900 2948 9d 2054 2991 10d Raman lines assigned to unreactecl tli\)oi,ane.
10 3 1 0 8 7 7 0
0 8d 0 0d
The Ramau frequencies of the solution of diborane in dimethyl ether a t the lower temperature (-70") are reported i n Table 111, along with the frequencies of the pure ether. TABLE I The effect of an increase in temperature on the spectrum was again a change in the relat,ive int,ensit.ies of some of tht. SOLUTIONS,CONCENTRATIONS A N D TEMPERATURES OF Rnman lilies. RAMAN DETERRIINATIONS Raman exposures were taken of the diborane-diethyl Concn. ether solutions, maintained a t -75O, until the background BIH~RzO (inole ratio) Temp., OC. on t,he phot,ographic film became too intense to permit the Soh. observation of any more Raman lines. The spectrum conB,H 6 4 4 H,O 1/4 -25 tained two weak frequencies, one a t 2689 and the othcr at 1/8 -25 2dfO em.-' which are not olmrvable in the spectra of the pure componentR. On the other hnnd, the spectrum of the B,He-( CH:i)rO 1/6 - 70 diborane-diethyl ether solution maintained a t 0" cont~ainetl - 10 1/6 only frequencies characteristic of the pure components. B2Ha-( C3Hj)sO 1/10 - 75 Discussion 0 1/10 (1) Taken in part froiii a thesis submitted b y Henry S. Uchida in partial fulfillnient of the requirements for t h e degree of Doctor of Philosophy, J u n e , 1954. (2) H. I. Rchlesinger and A. B. Burg, J. A m . Chem. Soc., 6 0 , 290 (1938); J. R. Elliott, W. L. Roth, G. F. Roedel and I. AI. Boldebuck, ihid., 7 4 , 5211 ( 1 9 5 2 ) . (3) R. C. Lord a n d E. Nielsen, J . O p t . Soc. A m . , 40, 65,5 (1950).
Raman Frequencies of the Borine Complexes.The spectral data were analyzed and found to be consistent with the assumption that et,her-horine complexes me formed. The Reman lines which cannot) be ascribed to the spectra of the pure components have been abstracted from the solution
appreciably from the (:orresponding bonds ill a cwmples in ~\.hicha boriiie group also is bonded to tmheoxygen. However, the solution spectra do SOIJL~TIOXA N I ) P U R E ~ihorniir-ditiiell~).Ictlier Diiiicthyl cllirrl not warraiit such a conclusion. I.'rrqiieni.y I ntcnsi t,.v E'recl w n c y In tpnsi t y There are noticeable similarities betweeii the (cm. -1j frequencies abstracted for the different complexes. 387 0 330 ( ? ) Where differences are observed, reasoiiable explana560 0 412 1 tions may be advanced for their occurrence. For 70 1;' n esa.mple, the arithmetic mean of the 387 and 560 87s 4 cm.-1 frequencies, which occur in the diborane02 1 B 020 5 dimethyl ether spectrum, is 473 cm.-'. This is 1044 1 almost identical with the 472 em.-' frequency of the 1092 1 1100 0 diborane-tetrahydrofuran spectrum. Later, in the 1144 1 1150 4 ,section on assignments of frequencies, this corre1171 2 spondence is made sigiiificaiit when a possibility for 1450 7 1448 6b Fermi resoiiance in the diboraiie-dimethyl ether 21O4lL 1 complex is proposed. 21 196 ? There are 110 frequencies in the diborane-di2290 4 methyl ether spectrum similar to the two that occur 2101 4 at 976 and 2914 cm.-l in the diborane-tetrahydro252A" 1 furan spectrum. The absence of the first line may 2812 10 2810 10 be simply A question of intensity. The diborane28U:j (i 2863 6 dimethyl ether spectrum had an intense continuous 202 1 3 2916 5 background which made the detection of weaker 295 1 (i 2996 1 4 lilies more difficult. The second frequency corre2092 0 2086 0 sponds t o an absorption band of the same magniR:tni:iii liiirs nssigiird to urweacted diborsaiich. t#udein the infrared spectrum of tetrahydrofuran spectre and are list'ed in Table 11'. There appears and therefore it may arise in the Raman spectrum to be it shift in the posit,ioiis of some of the lion-ab- of the complex. A similar conclusion may be stracted frequeiicies from t,lieir positioiis in the reached for t'he 1476 cm.-' frequency of the diborspectra of the pure ethers. Howe\w, t'he diffuse me-dimethyl ether spectrum which corresponds to nature of these Raman lines limited the precision st'rong absorption band in the infrared spectrum of with which t,lieir positioiis c~oultlbe measured and the pure ether. hlthough only two weak lilies were abstracted therefore it' \\-as notJ possi tile to ascribe any sigiiifimiice to these differences. The 1173 cm.-' fre- from the diborane-diethyl ether spectrum, they cluency of the t'eti,nliydt.ofui,atisolut,ioii is listed in also fit iiit,o the trend and correspond to the most abstracted frequencies in the Raman specour table of nbsti-acted frequeiicies ei'en t'hough it i~it~eiise is coiiicideiit with a frequeiicy in the spectrum of tra of the two other ether solutions. Assignment of Frequencies.-The R.aman fret,he pure ether. This was tloiie because t,here is a marked incarease i i i tflie relative int'eiisity of this qiiemies listed in Table I V are most, easily disfrequency in t,he spectrum of the solution as com- cussed in terms of a postulated molecular model for pared tBoits relatiire intelisity in t'he spectrum of the the R20:BH3complex. Consider the ether group as a point particle, X, att'ached to one of the termipure et.hei*. nal positions of a tetrahedral boroii atom. This TABLE I\postulated model for the H3BX molecule would beR,.411AN FREQIJENCIES O F ( I O M P L E X E S .kRSTRACTED FROM long to point group C3". All six normal modes of sPEC'TR.4 OF SOLI'TIONS vibration of this model give rise t o Raman active I j i horane-diethyl Diborane-te rraDi horn tie-di iiiet liyl ether Iiydrofuran ether fuiidamental frequencies. Most of the abstracted Frrilrrencirs Frequencies Preurienrirs (tin.-') frequencies (Table V) are assigned to normal modes 387 472 of vibratJion of this postulated molecular model of t'he ether-borine complexes. 560 850 TABLE 111
l{,.d h i .4 s
11' R P;Q I , E N ( s i i':h
)
P A
I.) IJI NTII P L ET[I E rt DINKTHYL ETHER
I)I B(
) RLV
E-
-1)
(~111.
(l.tl1. - 1 )
(l~lll:')
!)7(i
878
I049 11 i ; j 2281 2398 2994
1044
1171 1476 2290 240 1
r\SSIGShlENT O F
TABLE V FREQUENCIEB TO T H E H,BX
~~OLECULAR
IIODEL
2280 2110
It should be emphasized t.,at the abstract,ei frequencies do not representJ all the observed Raman frequeiicies of the addition complexes. The ether parts of the complexes gi\.e rise to frequencies so similar t,o t>lioseof the unreact,ed ethers tJhatJthese frequencies cannot be abstracted from the spectre of the solutions. One may have anticipated that the cnrhoii-oxygen bonds of an ether would differ ( 4 ) B. I,. Crowford, .Jr., a n d L. Joyce, J . Chom. P / I ~ / S7.,, 307 11939).
Designation VI
vt
va Y(
vj
Dewrilltion
B-H stretching (in phase) H-B-H &forination ( i n phase) R-0 Rt,retchiiig B-H stretching (out-of-phase) H-B-H deforinatioii
CIHSO: (CHs)?O: (CsHb)?O: Bns B H ~ B H ~
2281
2200
1049
1044
850 2:m
878 2401
1173
1171
(out~-of-~,h:rse) YR
HI=B-O heiidiiig
9if;
2280
2410
652
BERNARD RICEAND HENRYS. UCHIDA
There appears to be very little question about the assignment of the boron-hydrogen stretching frequencies, v1 and v4. The magnitudes of the frequencies assigned to V I are in good agreement with a frequency of 2290 cm.-l observed in the Raman spectrum of an aqueous solution of sodium borohydride5 and assigned to the symmetrical boron-hydrogen stretching mode of vibration of the borohydride ion. These frequencies are considerably lower than those characteristic of terminal hydrogen stretching in diborane and aluminum borohydride (2500 to 2600 cm.-l). I n the latter two ca,ses the terminal boron-hydrogen bonds are most likely of the sp2 type while in the ether-borine complexes and the borohydride ion they are probably sp3type bonds. The inteiise Raman lines at 859 and 878 cm.-l of tetrahydrofuran-borine and dimethyl ether-borine, respectively, were assigned to boron-oxygen stretching on the basis of the expected magnitude of the frequencies characteristic of bonds between atoms of such mass. The spectra.1data available for comparison with our boron-oxygen stretching frequency are limited ; however, comparable assignments a t 754 and 947 cm.-’ have been made for symmetrical and antisymmetrical boron-oxygen stretching, respectively, of the aqueous borate ioq6 B (OH), . Our assignment of frequencies to the BH3 deformation modes of vibration is consistent with simila r assignments in sodium borohydride’ aiid borim carbonyLM The weak Raman line that occurs at 97G cm.-l in the tetrahydrofuran-borine spectrum was assigned to H-B-0 bending; however, we do not have similar frequencies of other compounds for cornparison. The 172 cm.-l frequency abstracted from the tetrahyclrofuraii-borine spectrum was not assigned to the H,BX skeletal model but could be characteristic of B-0-C bending, while the 387 and 660 cm.-’ frequencies abstracted from the dimethyl etherborine spectrum may arise from a Fermi resonance between B-0-C and a C-0-C bending modes of vibratrion. As mentioned in the previous section, t,he 2994 cm.-’ frequency of tetrahydrofuran-borine and. the 1476 ern.-' frequency of dimethyl etherborine may arise from the ether parts of the complexes. Thus we account for all the abstracted frequencies. Of course, the possibility exists that reasonable assignments could be made for an entirely different molecular model. However, we were not, alile to find another model for which the frequencies could be assigned in ,z reasonable fashion. Potential Constants of Tetrahydofuran-Borine A normal coordinate treatment, based on the HaBX skeletal model, was carried out for tetrahydrofuran-horine. A similar treatment mas not macle for dimethyl ether-borine, 8nce one of the anticipated frequencies (h3)was missing. (5) W. J. Lehmann, P1i.D. Thesis, Saint Louis University, 1954. (6) J. 0. Edwards, G. C. Morrison, V. F. Ross and J. W. Schultz, J . A m . Chem. Soc., 77, 2GG (1955).
(7) W. C. Price, H. C. Longoet-Higgins, B. Rice and T. F. Toling, J . Chem. Phvs., 17, 217 (1949). (8) R. D. Cowan, i b i d . , 18, 1101 (1950).
Vol. 59
The valence force potential function used (similar to one proposed by Cleveland and Meister for methyl chlorideQ)was 2V = fdZ(Adi)2 f f ~ ( A o ) ’ 1L f a Z ( d A a i ) ’ f fpZ(dABi)* f 2fae[(dABl)(dA(~12f dAaza) f ( d A b ) ( d A a i z f dAaza) f (dAL%)(dAan f dAais)] f 2f~Z(dADAai)f 2f~pZ(dADApi)
In this equation d and D represent the boron-hydrogen and boron-oxygen distances, respectively, while a and /3 designate the hydrogen-boron-hydrogen- and hydrogen-boron-oxygen angles, respectively. The significance of the potential constants, the f’s in the equation, can be determined from the subscripts used. The values chosen for the bond distances were: d = 1.31 X cm. and D = 1.52 X cm. These values were supplied t o us by Dr. George W. Schaeffer of St. Louis University, who calculated them from covalent bond radii corrected for the difference in electronegativity of the bonded atoms. The angles a and /3 were assumed to be tetrahedral for lack of more precise information. A problem was presented in the choice of mass for the group represented by the symbol X. The valm obtained for the force constant fD, characteristic of the boron-oxygen bond, is very sensitive to the mass assigned. A minimum value forfD mould perhaps be obtained if the mass of an oxygen atom was used for X, while the result obtained if the entire tetrahydrofuran mass was used could be considered a maximum. Both masses were tried in separate calculations. Clearly, calculations of this type would be improved if it were possible to select, “ u priori,” an effective mass for a group treated as a point particle. The six frequency equations were solved by a method of approximation, whereby the force constants were adjusted until the left-hand and righthand sides of each equation differed by only a few tenths of a per cent. The results of our calculations are listed i n Table VI. As can be observed, the magnitude of the boron-oxygen stretching potential constant fD is very sensitive to the choice of mass of X. The only other term which differs appreciably in the two sets of values is ( f -~f ~ p~) . However, the significance of this “correction” term is not too apparent. Our lower value of 3.29 X 105 dyne cm.-’ for f~ appears to be closest to the “best” value. This can be seen by application of Gordy’s rule,I0 which relates force constants to the bond distanre and electronegativity of the bonded atoms. On the basis of this rule, a value of 3.71 X lo5 dynes cm.-’ is obtained for this force constant. We may consider this value as the magnitude of the force constant for a normal covalent boron-oxygen bond. Since our skeletal treatment of tetrahydrofurail-borine left us with a choice somewhere between 3.3 and 5.4 x 105 dynes cm.-’ for the boron-oxygen force constant, we can speculate that the donor-acceptor boron-oxygen bond in this molecule is comparable in strength to a normal covalent boron-oxygen bond. (9) F. F. Cleveland a n d A. G. Meister, “RIoleciilar Spectra TI.” Illinois Institute of Teclinology, 1948. (10) W. Uordy, J . Cirem. Phus., 14, .?OR (194fi).
July, 1955
THERMODYNAMICS OF SODIUM CARBONATE IN SOLUTION
G53
TABLE VI lations were not made for the diborane-diethyl ether system, since the observed lines characterisPOTENTIAL CONSTANTS OF BH3X MODELOF TETRAHYDROtic of the complex in this system were too weak. FURAN-BORINE, X lo6 DYNES CM.-1 We can conclude, therefore, that diethyl ether\[ass Of x f D fd fp fcYb f D a - fDB fa borine is the least stable of the complexes. X = 1G.00 3 . 2 9 3.01 0.300 0.474 0.0415 -0.320 X = 72.10 5 . 4 0 3.01 ,306 ,488 ,0406 - .502 TABLE VI1
Relative Stabilities of Ether-Borine Complexes A quaiititative measure of the apparent equilibrium constants was not attempted, but ail estimate can be made of the relative stabilities of the ether-bovines. The intensity, 1, of a Raman line characteristic of a particular substance is proportional to the concentration of that substance, hence
The ratio of the proportioiiality constants, k l / k z , should depend primarily on the nature of the two species in solution. The intensities of the lines a t 2524, 2284, 2290 and 2289 cm.-l were chosen as indicative of the concentration of diborane, tetrahydrofuran-borine, dimethyl ether-borine aiid diethyl ether-borine, respectively. If we assume that the ratios k l / k z are almost the same for all the diborane-ether systems investigated, a comparison of the stabilities of the complexes can be obtained. The above assumption appears to be reasonable, since the magnitude of the boron-hydrogen stretching frequency, whose intensity we have chosen to be indicative of the conceiitration of the complex, remains almost unchanged in the three ether-borines. Thus this bond is only slightly affected by the nature of the ether parts of the complexes and similarly the intensities of vibrations characteristic of this bond may be only slightly affected. The solutions for which we were able to calculate “iiitensity ratios” are listed in Table VII. Calcu-
“INTENSITY RATIOS”OF DIBORANE-ETHER SoLuiwNs Solution
B2Hs-CdHsO BsHe-( CH3)zO BzHs( CH3)zO
Initial mole ratio
B~HE/RzO
114 1/6 1/6
Teini)., “C.
1 ~ xI0 is/ ~
-25 -40 -70
IBtlls
S 2 4
The “intensity ratio” of dimethyl ether-boriiie a t -70” is twice as large as its value a t -40”. As anticipated, complex formation is favored by a drop in temperature. Even a t -25” the “iiiteiisity ratio” of the diborane-tetrahydrofuran system is four times as great as the value for the diboranedimethyl ether system a t -40”. It would be hest to compare “intensity ratios” of both solutions a t the same temperature and concentration of diborane. However, if the initial mole ratio of tliborane to tetrahydrofuran was reduced from I/’* to l / B aiid the temperature decreased from - 2 3 to - l O o , the “intensity ratio” for the diboimetetrahydrofuran system should he still greater than 8. Therefore, we can conclude that the tetrahydrofuran-borine complex is the most stable of tlic ether-borines. Thus the order of stabilities of the ether-l~oriiies is: C4HsOBH3> (CH&0BH3 > (CzH6)20BH3. This is the same order observedll for the snalogous ether-boron trifluoride complexes iii tlie vapor phase. This order emphasizes the predominaiice of steric to inductive effects. (11) H. C. Brown and R. M. Adtttiis, J . A m . Clrern. SOL,.,64, 2857 (1942).
THEIiMODYNL4MICS OF SODIUM CARBONATE I N SOLUTIOX’ BY
c. ED\\’4RD
Contribution froin The Institute
TAYLOR 01Paper Chetnistiy, Applcton, Wisconsin
Received Fsbruaty 1.4, 1066
The tlierinoiiynuinics of sodium c:trhon,tte in aqueous solution has been studied from 15 to 05’. From electromotive force measurements of the concentration cells 4 g - 4 g ~ c 0 . Na.C03( 1 0.1 ) NaHg Na2C03(m z )AgzCOa-Ag and determinations of solution vapor pressures, the activity co2Ticie:its have bean calculated over a concantralio:i range of 0.1 t o 1.5 m below 65’ and 0.1 to 2.5 ,n at 65’ and above. The relative partial inolal enthzlpies hzve been ca1oul:tteJ between 25 and 80”.
Experimental Concentration Cell Measurements.--The
equilibriuln c1.nl.f.s of the concentration cells were measured a t 15,25, 5 m and 37.5, 50 and 65’. The reference Sohtion ~ 3 0.1 drawn from a large reservoir of constantIn.composition The solu: The value of Inz \vas varied froln o.2 to tions mere prepared using weighed amounts of reagent grade sodium carbonate twice-distille,j The sodiulll carbonate analyzed 09.8 t o 09.9’% using constant-boiling hydrochloric acid diluted to o.5 after heating at 1400 for an hour. The dissolved oxygen in the solutions was ( 1 ) This communication contains material from a dissertation presrntcd t o T h e Institute of Paper Chemistry in partial fulfillment of tlie i,cijuircinents for the degree of Doctor of Philosophy froiii La.vrence College, J u n c , 1954. The work was done under the rlireutioii of Roy P. \V til tney.
removed by stripping with nitrogen a t reduced pressure, and the solutions were stored under a positive nitrogen pressure. The reference solution was stored in an 18-liter hottle lined lvith a bag to eliminate ntt,ack oll the glaRs by the solution. ~~~l~~~~ of each solution were run in triplicate using weighed samples and standard hydrochloric mid. The silver-silver carbonate electrodes were similar to the type 2 electrodes of Harned.2 Due t o the higher solubilitJy of silver carbonate, the electrodes were electrolyzed in 0.5 r n sodium bic:trbonate a t 200 nia./cm.a for two hours. It was necessary to form very fine porosity silver on the platinum spirals to obtain equilibrium potentials. These electrodes are apparently less stable than other silver electi,odes, but with care in preparation they were usually reproducible to 0.1 fnv. __(2) H.
S. Harned, J. A m .
Chain. S U C . ,Si, A l i i cl929).
