Instrumentation!

[Andiio]

Just a few things that need clearing up & confirmation!

1. What are the 'rules'/steps in identifying the number of proton/carbon environments?

2. Why does a higher chemical shift imply that the respective peak is bonded next to a more electronegative atom in NMR?

3. When asked for "the relative number of hydrogens" in each H-environment, is this simply indicated by the peak areas? i.e. if a peak has an area of 2H, would that indicate that it has 2 hydrogens?

4. In UV-Vis, what does it mean by a 'conjugated double/triple bond'?

5. Why does UV-Vis only analyse relatively low molecular weight substances? Also, can UV-Vis ever analyse metals/transition metals etc for the scope of the VCE course?

Thanks!

[burbs]

I'll give it my best shot

1) You mean how do you know two are in different environments? Well let's take CH3CH2CH2CH3 for example first. Now CH2 and CH3 are obviously different environments. The next step would be to see if the two CH2s and the two CH3s are in different environments. However, both are bonded to the same same things, as in both CH3s are bonded to one CH2, and both CH2 and bonded to one CH2 and one CH3. The low res for this would have 2 peaks.
Now if it was CH3CH2CH2OH, there are firstly 3 obvious groups here, the CH3, CH2 and the OH. However, one CH2 is bonded to a CH2 and a CH3, and the other is bonded to an OH and a CH2. Hence, these two are not identical, and are in different environments. So 4 peaks for this one. (not including TMS of course).

2) A little unsure exactly on this, but I would say that the more electronegative an atom, the more shielding experienced by the odd proton/neutron. Hence a greater magnetic field is needed to be applied.

3) I would say so, yes. Again not entirely unsure.

5) I would say because only the relatively low molecular mass substances absorb within the UV-vis range.

Hope that helped, take it  with a grain of salt.

[luken93]

1) I think burbs has pretty much explained it, there is no "official" method to do it.

2) In a nutshell, yeah. It's not much point looking into it too much, they can't ask you anything on spin-spin coupling so your knowledge is plenty...

3) Yep that's right. Peak height/area = equivalent hydrogens, peak splitting = n + 1 rule

4) Conjugation reduces the energy gap between the "excited" state of electrons and the normal state. In other words, it takes less energy (and is easier) to excite electrons in compounds that have conjugated double bonds. Hence, the more double bonds, peaks will occur at higher wavelengths as it takes less energy to excite them. Don't think this is needed though, where did you hear/see this?

5) I'll wait for someone better to answer this...

[Mao]

2. Electrons shield the nucleus. Electron-withdrawing groups (electronegative elements) withdraws electron density from the nucleus, 'de-shielding' the nucleus, thus changing the chemical shift. If you know a fair bit of electromagnetism, you can take a read of this: http://en.wikipedia.org/wiki/Chemical_shift. But primarily, don't worry about it, it's not part of VCE.

4. Conjugated systems are alternating single/double bonds. Such as 1,3-butadiene, 1,3,5-hexatriene, etc. These systems have 'delocalised' electron clouds similar to benzene. luken93 explains why this lowers the transition energy.

5. I don't see why not, you would simply need to understand what the calibration curve means in terms of determining concentration from a set of standards.

[luken93]

So for 5) Mao, if it were to be a MC Q on which method could be used you could say UV-Vis if there were no other option?

[Mao]

So for 5) Mao, if it were to be a MC Q on which method could be used you could say UV-Vis if there were no other option?

Highly doubt it will be on an exam. Metal complexes aren't in the syllabus.