[Big-Daddy]

With NMR spectra which are based on isotopes other than ¹H (ranging from ¹³C to ¹⁵N etc.) - and dealing with benzene or phenyl groups separately as I know exceptions crop up - is it reasonable to say that the number of bonds the main atom (i.e. the one for which the spectra are named) has to other atoms of the same type decides how many peaks there are? (Obviously, double bonds count as 2 bonds, etc.)

I.e. whatever the arrangements, let's say you have 3 C atoms with 2 C bonds each and 2 H bonds each; 2 C atoms with bonds to 3 C atoms each and one H bond; and 1 C atom with bonds to 3 H atoms and 1 C bond. (Whether this structure can be real or not isn't relevant to this discussion; I just want to know about the counting.) Then does this mean there will be 3 peaks, 1 with a height/area of 3 (for those with 2*C-C+2*H-H), one with a height/area of 2 (those with 3*C-C+1*H-H) and one with an area of 1 (the one with 1*C-C+3*H-H)? Regardless of the arrangement? (i.e. it's all about how many of each atom they're connected to, with double bonds obviously making the environment instantly different from single bonds.)

I accept that electronegative atoms may distort this. However, is it true for a compound with just two elements? (e.g. the one I described above, or one with just N and H, or one with just P and S)

Furthermore, I have never encountered any splitting patterns on non-¹H NMR spectra. How come?

[Babcock_Hall]

With respect to splitting, there are two main reasons I can think of.  In carbon spectra, each C-13 nucleus has approximately 99% chance of being bonded to a C-12 nucleus, which won't show splitting.  On the other hand, if you have, for example, 100% C-13 labeled acetic acid, the two carbon atoms will show J-coupling.  The second reason is that some molecules only have one nucleus of a given type.  For example serine phosphate has only one P-31 nucleus, so will not be any P-31/P-31 splitting.  On the other hand, adenosine triphosphate (ATP) does show J-coupling among the three phosphorus nuclei.

I did not entirely follow the rest of your comment.  However, I would say that there will be as many peaks as there are groups of equivalent nuclei.  For example, there are two signals for acetic acid in a C-13 spectrum and three signals for ATP in a P-31 NMR spectrum.

[discodermolide]

Usually in for example a ¹³C-NMR the proton coupling is removed by irradiation at the frequency of the signal, known as off-resonance, this gives a single line for the ¹³C signal. It is the same for the other NMR active atoms. The rest of your post I did not understand.

[Big-Daddy]

With respect to splitting, there are two main reasons I can think of.  In carbon spectra, each C-13 nucleus has approximately 99% chance of being bonded to a C-12 nucleus, which won't show splitting.  On the other hand, if you have, for example, 100% C-13 labeled acetic acid, the two carbon atoms will show J-coupling.  The second reason is that some molecules only have one nucleus of a given type.  For example serine phosphate has only one P-31 nucleus, so will not be any P-31/P-31 splitting.  On the other hand, adenosine triphosphate (ATP) does show J-coupling among the three phosphorus nuclei.

I did not entirely follow the rest of your comment.  However, I would say that there will be as many peaks as there are groups of equivalent nuclei.  For example, there are two signals for acetic acid in a C-13 spectrum and three signals for ATP in a P-31 NMR spectrum.

What I mean is: take a look at the structure of adamantane. It has 2 elements - C and H - and its C atoms fall under just 2 categories: those with bonds to 3 Cs and 1H, or 2 Cs and 2 Hs. This means (I know this for a fact in this particular case) that there are just 2 peaks in the 13-C NMR of adamantane.

As an extension, it is reasonable to say that for any molecule with just 2 elements, the two considerations will be: a) the type of bond (single, double and triple lead to different environments and thus different peaks) and b) how many Cs a particular atom is bonded to versus how many Hs they are bonded to, for example (i.e. how many atoms of each element they are bonded to). If two atoms are identical with regards to both a) and b) above, will they then fall under the same peak?

[Babcock_Hall]

I think that the key question is what constitutes isochronous nuclei (nuclei having the same chemical shift).  As far as I am concerned, the same rules apply for all NMR-active nuclei.  In the case of adamantane, there are only two kinds of carbon atoms on the basis of symmetry arguments.  The same is true of diethyl ether.

Consider beta-carotene (C₄₀H₅₆), which has only carbon and hydrogen atoms.  It has 19 C-13 signals (it has symmetry that makes carbon atoms on either end isochronous).  The pairs of carbon atoms that are involved with double bonds all have unique chemical shifts (5 and 5' through 15 and 15'), even though 15 has the same number of hydrogens as 11, for example.
http://lipidbank.jp/cgi-bin/detail.cgi?id=VCA0001

[Big-Daddy]

I think that the key question is what constitutes isochronous nuclei (nuclei having the same chemical shift).  As far as I am concerned, the same rules apply for all NMR-active nuclei.  In the case of adamantane, there are only two kinds of carbon atoms on the basis of symmetry arguments.  The same is true of diethyl ether.

Consider beta-carotene (C₄₀H₅₆), which has only carbon and hydrogen atoms.  It has 19 C-13 signals (it has symmetry that makes carbon atoms on either end isochronous).  The pairs of carbon atoms that are involved with double bonds all have unique chemical shifts (5 and 5' through 15 and 15'), even though 15 has the same number of hydrogens as 11, for example.
http://lipidbank.jp/cgi-bin/detail.cgi?id=VCA0001

Okay, so what are these rules (exceptions not being considered)?

Two of them I have predicted (i.e. two differences which can, on their own, cause two C nuclei to bring about different peaks due to being in different environments):

1) The bonding of the nuclei (e.g. 4 single bonds as opposed to 2 single bonds and 1 double bond).

2) The atoms to which the nuclei are bonded (e.g. being bonded to 3 C and 1 H atom leads to a different environment from being bonded to 1 C and 3H atoms or 2C and 2 H atoms).

What other factors, obviously determinable from the molecule structure, can be considered?

[Babcock_Hall]

It seems to me that the conditions you mentioned are sufficient, but not necessary to cause the environments of two nuclei to be different.  The example of beta-carotene was meant to show that two C-13 nuclei could have identical hybridization and number of attached H nuclei and still have different chemical shifts.

Here is what Silverstein (4th edition, pp. 200-201) says:  "If nuclei are interchangeable by a symmetry operation or a rapid process, they are chemical shift equivalent (isochronous); that is, they have exactly the same chemical shift under all achiral conditions.  Nuclei are interchangeable if the structures before and after the operation are indistinguishable....The symmetry element (axis, center or plane) must be a symmetry element for the entire molecule."  Does this help?

[Big-Daddy]

It seems to me that the conditions you mentioned are sufficient, but not necessary to cause the environments of two nuclei to be different.  The example of beta-carotene was meant to show that two C-13 nuclei could have identical hybridization and number of attached H nuclei and still have different chemical shifts.

Here is what Silverstein (4th edition, pp. 200-201) says:  "If nuclei are interchangeable by a symmetry operation or a rapid process, they are chemical shift equivalent (isochronous); that is, they have exactly the same chemical shift under all achiral conditions.  Nuclei are interchangeable if the structures before and after the operation are indistinguishable....The symmetry element (axis, center or plane) must be a symmetry element for the entire molecule."  Does this help?

Well, I'm not entirely sure what is meant by "symmetry element".

[discodermolide]

Quote"Symmetry element
A symmetry element is a point of reference about which symmetry operations can take place. In particular, symmetry elements can be centers of inversion, axes of rotation and mirror planes. Wikipedia"

[Babcock_Hall]

There is another thread right now that might help you.  They posted a link that discusses symmetry, among other topics:
http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-08-symmetry.htm

[Big-Daddy]

Quote"Symmetry element
A symmetry element is a point of reference about which symmetry operations can take place. In particular, symmetry elements can be centers of inversion, axes of rotation and mirror planes. Wikipedia"

Wikipedia references are hardly likely to help me (a pre-undergraduate) understand anything.

I was asking a question about what sort of occurrences constitute symmetry elements in chemistry, with regards to large molecules/

[Babcock_Hall]

Big-Daddy, are you familiar with phrases such as "plane of symmetry" or "mirror plane?"